Optics B


Reflection, specular and diffuse:

Reflection: Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated.


Specular: Reflection off of smooth surfaces such as mirrors or a calm body of water leads to a type of reflection known as specular reflection. 

Diffuse: Reflection off of rough surfaces such as clothing, paper, and the asphalt roadway leads to a type of reflection known as diffuse reflection. Whether the surface is microscopically rough or smooth has a tremendous impact upon the subsequent reflection of a beam of light. The diagram below depicts two beams of light incident upon a rough and a smooth surface.


Law of Reflection:

The law of reflection states: The angle of incidence of a wave or stream of particles reflecting from a boundary, conventionally measured from the normal to the interface, is equal to the angle of reflection, measured from the same interface.

Refraction, Index of refraction, Snell’s law, and Critical Angle:

Refraction: Refraction is the bending of light (it also happens with sound, water and other waves) as it passes from one transparent substance into another. This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms and rainbows.   

       

Formula is :


Index of refraction:  , n= refractive index, c = speed of light in a vacuum, v = velocity of light in the given medium. The phase velocity of light in a vacuum is 299,792,458 m/s.

Examples of the speed of light:

Speed of light in air: 299,705kms/sec

Speed of light in water: 225,000kms/sec

Speed of light in glass: 200,000kms/sec

Speed of light in diamond: 125,000 kms/sec

The refractive index is important because when light goes from a medium with a higher refractive index to lower the light speeds up and bends away from the normal line. And vice-versa happens with materials in a lower refractive index which slow down and bends toward the normal line. This means that objects with a higher refractive index are more refractive.

Critical Angle: The critical angle is the angle of incidence when the angle of refraction is 90 degrees. This has to happen from a more dense to less dense medium.




Prisms(dispersion): When light falls on the glass prism, refraction takes place. Since the wavelength of different components of light is different and the frequency is constant, each component gets deviated by a different angle due to the difference in velocity in the glass medium.

Prisms(deviations): The angle of deviation is the angle between the original path that the beam should have followed and the deviated beam. It can also be defined as the angle through which the incident ray gets deviated when passing through a prisms



Prisms (deviation & dispersion, etc):

Prisms(dispersion): When light falls on the glass prism, refraction takes place. Since the wavelength of different components of light is different and the frequency is constant, each component gets deviated by a different angle due to the difference in velocity in the glass medium.

Prisms(deviations): The angle of deviation is the angle between the original path that the beam should have followed and the deviated beam. It can also be defined as the angle through which the incident ray gets deviated when passing through a prism.



  • Refraction is when light passes through different mediums. Light can change speeds while traveling through different media and the optical density of a medium measures how well it can transmit light, the higher density the slower the water. The index of refraction is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium. An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light.




This is an image of a prism that is a triangular prism which is the most popular shape for a prism. Two sides of the prism can’t be parallel or else it isn’t a prism. There are two types of prisms: deviation/reflection/refraction and dispersion. Deviation prisms are used to reflect light to displace or deviate the light beam. They are used to erect the image in binoculars and single-lens reflex cameras. Dispersion prisms are used to disperse the light into many different colors.




Some Types of Prisms:

Porro prism

Porro–Abbe prism

Amici roof prism

Pentaprism and roof pentaprism

Abbe–Koenig prism

Schmidt–Pechan prism

Bauernfeind prism

Dove prism

Retroreflector prism






Mirrors/Lenses:





  • There are two main types of lenses and mirrors: convex and concave. Convex mirrors are known as diverging mirrors because the incident rays diverge upon reflection. Similarly, concave mirrors are called converging mirrors and lenses because the incident rays converge upon reflection. In lenses, the opposite is true. Convex lenses are converging lenses, and concave lenses are known as diverging lenses. There are six types of lenses that fall under those two categories. The six lenses types are: plano-convex, plano-concave, double convex, double concave, concavo-convex, and convexo-concave. Mirrors reflect, and lenses refract.

    • Aperture: Describes how much light will be intercepted by the mirror.

    • Center of curvature: Useful when locating images, the center of the imaginary sphere upon which a curved mirror rests. The flatter the mirror, the farther away the center of curvature.

    • Concave mirror: Converging mirror which forms either real or virtual images which may be magnified.

    • Convex mirror: Diverging mirror which forms virtual images which may be magnified.

    • Focal length: The distance from the lens or mirror to the principal focus.

    • Principal axis: The main line drawn through the center of the mirror or lens upon which information such as center of curvature or principal focus is given.

    • Principal focus: The point where light rays parallel and close to the principal axis converge or appear to diverge.

    • Ray diagram: A tracing of the light rays to show where the image forms after being reflected off a mirror or refracted through a lens.

    • Real image: An image that can be projected onto a screen at its location, inverted relative to the object, and can be magnified.

    • Secondary axis: Any line drawn through the center of curvature to the mirror or lens.

    • Vertex: The center of a lens or mirror.

    • Virtual image: An image that cannot be projected onto a screen, is not inverted relative to an object, and can be magnified.














Ray-Tracing:




  • Trial 1: (Concave Mirror) The object is located beyond the center of curvature. The image is located between the center of curvature (C) and the principal focus (F). It is reduced in size and inverted. The image is real.

  • Trial 2: (Concave Mirror) The object is located at the center of curvature. The image is located at the center of curvature. It is inverted. The image is real.

  • Trial 3: (Concave Mirror) The object is located between the center of curvature and the principal focus. The image is located beyond the center of curvature. It is enlarged in size and inverted. The image is real.

  • Trial 4: (Concave Mirror) The object is located at the principal focus. No image is formed. All rays are reflected from the mirror as parallel rays.

  • Trial 5: (Concave Mirror) The object is located between the principal focus and the mirror. The image appears to be located behind the mirror. It is enlarged in size. The image is virtual.

  • Trial 6: (Convex Mirrors) All convex mirrors form a virtual image reduced in size.

  • On Trial 4, the ray that does not go through the center of curvature should go through the principal focus.

  • Finding approximate image locations with lenses is fairly similar to finding image locations with mirrors. The only difference is that instead of having a center of curvature, the important point is listed as "2F" or twice the focal length.

  • How to find approximate image locations while using lenses


  • Trial 1: (Convex Lens) The object is located beyond twice the focal length. The image is located between the focal length and twice the focal length on the opposite side of the lens. It is reduced in size and inverted. The image is real.

  • Trial 2: (Convex Lens) The object is located at twice the focal length. The image is located at twice the focal length on the opposite side of the lens. It is inverted. The image is real.

  • Trial 3: (Convex Lens) The object is located between twice the focal length and the focal length. The image is located beyond twice the focal length on the opposite side of the lens. It is enlarged in size and inverted. The image is real.

  • Trial 4: (Convex Lens) The object is located at the principal focus. Just like mirrors, no image is formed at this position. All rays are refracted from the lens as parallel rays.

  • Trial 5: (Convex Lens) The object is located between the principal focus and the lens. The image appears to be located behind the object on the same side of the lens. It is enlarged in size. The image is virtual.

  • Trial 6: (Concave Lenses) All concave lenses from a virtual image reduced in size.





























Mirrors and lenses facts:

Mirrors & Lenses(convex): 

A convex mirror, also known as a curved mirror, has a reflective surface that bulges towards the light source. Because a convex mirror reflects light outwards, it cannot be utilized to focus light. A convex mirror is also known as a diverging mirror because when light strikes its reflecting surface, it diverges.

A convex mirror forms only virtual,diminished and erect image for all position of object ,where as convex lense can form both real,inverted images of various size and virtual,erect and large images depending on the position of object

Top: Lens, Bottom: Mirror



Mirrors and lenses(concave): 

Concave lenses have at least one surface curved inside. A concave lens is also known as a diverging lens because it is shaped round inwards at the center and bulges outwards through the edges, making the light diverge. They are used to treat myopia as they make faraway objects look smaller than they are.

A concave mirror is a curved mirror where the reflecting surface is on the inner side of the curved shape. It has a surface that curves inward, resembling the shape of the inner surface of a hollow sphere.




Mirrors and Lenses(plain): 

Plane Mirrors Study Guide | Inspirit

A plane mirror is a smooth or highly polished surface that reflects the light or wave for the formation of an image. It is a flat, polished, and reflective surface that produces a virtual image of the real object. Pretty Much a normal mirror.































Types of Magnification:

Real and Virtual objects:

A virtual object or image is just a way of describing what's happening to the light because neither can have any effect in itself. A real object actually has light coming from it and a real image actually has light actually passing through it. This is why people say that you can't project a virtual image.


Finding the Focal point:









Erect and Inverted Images:

Inverted image is on which the image formed is upside-down by the object. Erect image is not inverted but it is straight as if the object.



Magnification:

The term magnification means a comparison between the size of the image formed by a lens with respect to the size of the object. For a lens: Magnification 'm' is the ratio of the height of the image to the height of the object.


































Color Theory:

=Additive & subtractive color:





Because additive colors use transmitted light, the colors appear much brighter and create a larger visible spectrum, producing millions of colors on a screen. Subtractive colors use reflected light, so they appear muted in contrast.



Primary and secondary colors:

Three Primary Colors (Ps): Red, Yellow, Blue.

Three Secondary Colors (S'): Orange, Green, Violet.




Absorption and Reflection:

The colours we see are the wavelengths that are reflected or transmitted. For example, a red shirt looks red because the dye molecules in the fabric have absorbed the wavelengths of light from the violet/blue end of the spectrum. Red light is the only light that is reflected from the shirt.

LIGHT:


LIGHT:


7

Additive color or additive mixing is a property of a color model that predicts the appearance of colors made by coincident component lights, i.e. the perceived color can be predicted by summing the numeric representations of the component colors.



Subtractive color or subtractive color mixing predicts the spectral power distribution of light after it passes through successive layers of partially absorbing media.



RGB to HEX code: (x+y/16) x is the answer to the R/16 and y is the remainder of the R/16

(w+t/16) w is the answer to the G/16 and t is the remainder of the G/16

(z+s/16) z is the answer to the R/16 and s is the remainder of the R/16

Xywtzs is the hex code and if any of the numbers are greater than 9 then is will be 10=A 11=B 12=C and so on



HEX code to RGB is the opposite: You’ll figure it out




Anatomy of EYE:






Structure and function of the human eye:



Sclera: the dense fibrous opaque white outer coat enclosing the eyeball except the part covered by the cornea. called also sclerotic, sclerotic coat.



Iris: In humans and most mammals and birds, the iris ( pl. : irides or irises) is a thin, annular structure in the eye, responsible for controlling the diameter and size of the pupil, and thus the amount of light reaching the retina.




Cornea: The cornea is the transparent part of the eye that covers the front portion of the eye. It covers the pupil (the opening at the center of the eye), iris (the colored part of the eye), and anterior chamber (the fluid-filled inside of the eye). The cornea's main function is to refract, or bend, light.



The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina. It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye that mostly miss exiting the narrow pupil.



Lens: lens, in anatomy, a nearly transparent biconvex structure suspended behind the iris of the eye, the sole function of which is to focus light rays onto the retina.

Ciliary body and muscle: The ciliary body is a circular structure that is an extension of the iris, the colored part of the eye. The ciliary body produces the fluid in the eye called aqueous humor. It also contains the ciliary muscle, which changes the shape of the lens when your eyes focus on a near object.



Conjunctiva: The conjunctiva is a thin, clear membrane that protects your eye. It covers the inside of your eyelid and the white of your eye (the sclera). The conjunctiva creates the mucus layer that forms part of your tears.



Retina: The retina is the light-sensitive layer of tissue at the back of the eyeball. Images that come through the eye's lens are focused on the retina. The retina then converts these images to electric signals and sends them along the optic nerve to the brain.



Optic nerve: The optic nerve is comprised of millions of nerve fibers that send visual messages to your brain to help you see. You have an optic nerve at the back of each eye that connects directly to your brain.



Macula: The macula is part of the eye's retina. Located in the center of the retina, the macula is responsible for giving us clear vision and the ability to see fine detail. Although it is small, it enables us to see "20/20" and to see our world in color.



Retinal Blood Vessels: The retinal blood vessels are the central retinal artery and vein, and their branches. The arteria centralis retinæ and its accompanying vein pierce the optic nerve, and enter the bulb of the eye through the porus opticus.



Vitreous body: The vitreous body (vitreous meaning "glass-like"; from Latin vitreus 'glassy', from vitrum 'glass', and -eus) is the clear gel that fills the space between the lens and the retina of the eyeball (the vitreous chamber) in humans and other vertebrates.



Disease of the eye

Myopia: nearsightedness; rays converge in front of the retina. Corrected with a diverging lens. The 

Hyperopia: farsightedness; rays converge behind the retina. Corrected with a converging lens



Astigmatism: irregularly shaped cornea; in some cases caused by lens (lenticular astigmatism). Corrected with eyeglasses; astigmatism causes near and farsightedness.



Cataract: Doesn’t cause pain, redness, or tearing. When the inner lens of the eye becomes darkened or opaque, the condition is called a cataract. The lens may be surgically replaced with a plastic lens. The implanted lens is of fixed focal length, so it is not capable of accommodation like the natural lens. This is usually not a major concern, because persons who develop cataracts after age 60 do not have much accommodation remaining because the inner lens has become less pliable with age.



Aphakia: Aphakia is the absence of the lens of the eye. It causes a loss of accommodation, farsightedness (hyperopia), and a deep anterior chamber. Complications include detachment of the vitreous or retina, and glaucoma.



Glaucoma: A group of diseases that damages the optic nerve. This can be due to an infection or other critical conditions and may lead to blindness if not cured early. It is due to a build-up of pressure in the eye.



Presbyopia: Presbyopia is a condition associated with aging of the eye that results in progressively worsening ability to focus clearly on close objects. Presbyopia is a natural part of the aging process. It is due to hardening of the lens of the eye causing the eye to focus light behind rather than on the retina when looking at close objects



Amblyopia: A type of eye disease that is commonly referred to as a "lazy eye". This is generally due to a nerve pathway being damaged in the brain during childhood and may lead to blindness if not treated early.




Rods are responsible for vision at low light levels (scotopic vision). They do not mediate color vision, and have a low spatial acuity. Cones are active at higher light levels (photopic vision), are capable of color vision and are responsible for high spatial acuity.
















Structure and function of microscopes, telescopes, cameras, glasses, retro reflectors, and periscopes:

How do microscopes work?

Most microscopes use lenses to magnify the image of an object by focusing light or electrons



Microscopes

Devices that produce magnified images of structures that are too small to see with the unaided eye



Telescope

A device built to observe distant objects by making them appear closer



How do telescopes work?

Telescopes work by using lenses to focus light and mirrors to reflect the light



Telescope lens

Refraction of light rays through a lens or the reflection of light rays by a concave mirror



Microscope lens

Convex lens bends light to make the object appear larger



Camera

An optical instrument that uses lenses to focus light, and film to record an image of an object.



How do cameras work?

Cameras focus light like your eye and record the image with a detector



Camera lens:

 Optical lens or assembly of lenses essential for forming an image on a camera sensor.



How do periscopes work?

Uses a system of prisms, lenses or mirrors to reflect images through a tube.



Glasses:

They refract the light rays to focus on the retina

For myopia they have concave lens

For hyperopia convex lens

































Absorption in spectrums:




Radio Waves







Glossary:

How do microscopes work?

Most microscopes use lenses to magnify the image of an object by focusing light or electrons




Microscopes

Devices that produce magnified images of structures that are too small to see with the unaided eye




Telescope

A device built to observe distant objects by making them appear closer




How do telescopes work?

Telescopes work by using lenses to focus light and mirrors to reflect the light




Telescope lens

Refraction of light rays through a lens or the reflection of light rays by a concave mirror




Microscope lens

Convex lens bends light to make the object appear larger




Convex lens

A piece of transparent glass curved so that the middle is thicker than the edges




Refraction of light

The bending of light as it passes from one medium to another




Camera

An optical instrument that uses lenses to focus light, and film to record an image of an object.




How do cameras work?

Cameras focus light like your eye and record the image with a detector




Camera lens

Optical lens or assembly of lenses essential for forming an image on a camera sensor.




Optical lens

Type of zoom that uses a camera lens to bring the subject closer.




Focal length

The distance from the center of a lens to the focal point




Focal point

The center of interest on a page or set of facing pages, created by using color, contrast and proportion.




Glasses

Optical instrument consisting of a pair of lenses for correcting defective vision




How do glasses work?

Light passing through is refracted towards the thickest part of the lens




They refract the light rays to focus on the retina

For myopia they have concave lens

For hyperopia convex lens

Glasses




Myopia

Nearsightedness




Hyperopia

Farsightedness




Astigmatism

A condition in which the eye does not focus properly because of uneven curvatures of the cornea




Uneven curvature of the cornea

Astigmatism




Retro reflectors

An optical device in which the emergent ray is parallel to the incident ray




Emergent ray

The light ray that leaves a lens after refraction




Incident ray

The ray that comes from an object and strikes a surface.




Optical device

Any instrument or object used to produce or control light




Periscopes

Used in the trenches to see over the top of the trench without being shot




How do periscopes work?

Uses a system of prisms, lenses or mirrors to reflect images through a tube.




Prisms

An object that separates white light into the colors that make it up




Real images

A copy of an object formed at the point where light rays actually meet




Virtual images

An optical image formed from the apparent divergence of light rays from a point, as opposed to an image formed from their actual divergence.




Example of real image

Projector




Example of virtual image

Reflection (mirror)



For the water-air boundary, the critical angle is 48.6-degrees. For the crown glass-water boundary, the critical angle is 61.0-degrees. The actual value of the critical angle is dependent upon the combination of materials present on each side of the boundary.




How can Jeff see the pretty colors on the readable side of a CD? What is a CD an example of? (1 point)

CDs have small pits in the plastic in a spiral, wound tightly enough to caught the light to diffract into a full visible

spectrum (example of diffraction grating)



What scenario(s) with one mirror produces a real, inverted image:

Concave mirror anywhere except at the primary focal point (f) (1 point)




















## Structure of a Sextant






A sextant is a navigation instrument used to measure the angle between two visible objects, typically the horizon and a celestial body like the Sun, Moon, or stars. The structure of a sextant includes several key components:



- Frame: Provides the structural base for the sextant, shaped as a sector of a circle, spanning 60 degrees, hence its name[1][2].

- Arc: A graduated scale marked in degrees, used to measure angles. The arc itself spans 60 degrees, but the scale can measure up to 120 degrees[4][5].

- Index Arm: A movable arm pivoted at the center of the arc, which holds the index mirror. It is used to adjust the angle of the reflected image[3][7].

- Index Mirror: Attached to the index arm, this mirror reflects the light from celestial bodies towards the telescope[4][5].

- Horizon Mirror: A half-silvered mirror that allows the user to see both the direct image of the horizon and the reflected image of the celestial body[4][5].

- Telescope: Used to observe the horizon and the reflected image of the celestial body[1][5].

- Micrometer Drum and Vernier Scale: These provide precise measurements of the angle in minutes and seconds of arc[5][8].

- Release Levers and Clamp: Allow the index arm to be locked in place or moved freely along the arc[1][7].



## Function of a Sextant



The primary function of a sextant is to measure the angular distance between two visible objects, typically the horizon and a celestial body. This measurement is crucial for celestial navigation, allowing navigators to determine their latitude and longitude.



### How It Works



1. Alignment: The user adjusts the index arm so that the reflected image of the celestial body aligns with the direct image of the horizon through the telescope[4][5].

2. Angle Measurement: The angle between the horizon and the celestial body is read from the graduated arc. This angle is known as the altitude[3][5].

3. Navigation: By combining the measured altitude with the time of measurement and data from almanacs, navigators can calculate their position on a chart[3][5].



### Uses



- Celestial Navigation: To determine latitude and longitude by measuring the altitude of celestial bodies[3][6].

- Terrestrial Navigation: To measure horizontal angles between landmarks for position fixing or to calculate distances using trigonometry[6][4].



## Correction of Optical Problems in Human Eyes Using Lasers



Laser vision correction is a revolutionary approach to treating refractive errors such as myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. This technology has evolved significantly over the years, offering various techniques tailored to individual needs.



### Types of Laser Vision Correction



1. LASIK (Laser-Assisted In Situ Keratomileusis)

   - Procedure: A thin flap is created in the cornea using either a microkeratome or a femtosecond laser. The flap is then lifted, and an excimer laser reshapes the underlying corneal tissue to correct vision. The flap is replaced, and it heals naturally without stitches[2][3].

   - Advantages: Quick recovery, minimal discomfort, and high success rate.

   - Variations: 

     - Wavefront LASIK: Uses a 3D map of the cornea to correct higher-order aberrations, providing more precise vision correction[2].

     - Contoura Vision: Utilizes corneal topography for highly customized correction, addressing irregularities in the cornea[4].



2. PRK (Photorefractive Keratectomy)

   - Procedure: The outer layer of the cornea is removed, and an excimer laser reshapes the underlying tissue. The outer layer regenerates over time[5].

   - Advantages: Suitable for patients with thin corneas.

   - Disadvantages: Longer recovery time and potential discomfort during healing.



3. LASEK (Laser Epithelial Keratomileusis)

   - Procedure: A thin flap is created using an alcohol solution, and the cornea is reshaped with a laser. The flap is replaced and secured with a bandage contact lens[5].

   - Advantages: Suitable for patients with thin corneas.



4. SMILE (Small Incision Lenticule Extraction)

   - Procedure: A minimally invasive, flapless procedure where a small incision is made to remove a lenticule of corneal tissue, reshaping the cornea[10].

   - Advantages: Less invasive, potentially fewer complications.



### How Lasers Correct Optical Problems



Lasers in vision correction primarily work by reshaping the cornea, which is the clear front surface of the eye. The cornea plays a crucial role in focusing light onto the retina. In eyes with refractive errors, light is not focused correctly, leading to blurred vision.



1. Principle: The excimer laser, used in most procedures, emits ultraviolet light pulses that remove microscopic amounts of tissue from the cornea, reshaping it to improve the way light enters the eye[3].



2. Customization: Advanced techniques like wavefront-guided and topography-guided LASIK use detailed maps of the eye to correct unique imperfections, providing personalized vision correction[7].



### Safety and Effectiveness



- Safety: Laser vision correction is generally safe, with low complication rates. The use of advanced technologies like femtosecond lasers has further reduced risks[1][2].

- Effectiveness: Most patients achieve significant improvement in vision, with many attaining 20/20 vision or better[2][6].



### Conclusion



Laser vision correction offers a range of effective solutions for treating refractive errors, each with its advantages and specific applications. By understanding the different techniques and technologies involved, individuals can make informed decisions about their vision correction options.




Make Sure to make lines go on infinitely(draw ←—→ on the ends of the lines







Convex lenses(Object is behind the focal point):

Draw the ray parallel to the PA from the object, it goes through the focal point.

Draw the ray through the focal point, it is parallel to the lens.

Draw one line through the center from object to image(where the two other lines intersect).























Convex lens(image is in front of the focal point):

Draw the ray parallel to the PA, from the object, it goes through the focal point.

Draw an imaginary line(dotted) from the focal point to the object. Then make the ray, following the same path as the previous one(through the focal point) to the lens, then the line should end up being parallel to the PA.



Next extend the versions of the two lines that have been refracted by the lens, they should meet at the image.



Draw the green line through center, and the part of the line behind the object has to be dotted.















Concave lens works for both behind and before focal point:

Draw line parallel to the PA from the object, and then to find where the line is refracted, draw an imaginary line(dotted) from the ←focal point to where the previous ray meets the lens, and then continue this line as non-imaginary as the refracted ray.

Draw a line to the → focal point from the object, and then continue it as dotted, until it reaches the focal point. Once the line/ray passes through the lens, it becomes parallel to the PA. Draw the rest of the refracted ray as a dotted line.

Draw line from the object, through the center, and to infinity and beyond.

Where the solid green line, and the dotted red/blue lines meet is where the image is.


















Plane mirrors:

Dobject is the distance between the object and the mirror.

Top of Person’s head  is the object

Draw a line Parallel of the object, and a dotted line from where it seems to be coming from(other side of mirror)

Draw a line at angle x(any angle) from the object going down, and how it reflects off the mirror(angle of incidence = angle of reflection), and a dotted line from where the reflected ray seems to be coming from.

Draw a line at the same angle x(same from previous step) from the object going up, and how it reflects off the mirror(angle of incidence = angle of reflection), and a dotted line from where the reflected ray seems to be coming from.

Where all the dotted lines meet, is where the image forms(always virtual).