Ray Optics (Theory)
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57 Terms
What is ray optics?
Ray optics is the branch of optics in which light is treated as travelling in straight lines called rays. It explains reflection, refraction and image formation when the wavelength of light is much smaller than the size of the obstacle.
State the laws of reflection.
- The incident ray, reflected ray and normal lie in the same plane.
- Angle of incidence = Angle of reflection.
Define: (a) Pole
The geometrical centre of a spherical mirror.
Define: (b) Centre of curvature
The centre of the sphere of which the mirror is a part.
Define: (c) Radius of curvature
Distance between pole and centre of curvature.
Define: (d) Principal axis
The line joining pole and centre of curvature.
Define: (e) Principal focus
The point where rays parallel to the principal axis meet (or appear to diverge from) after reflection.
What is the relation between focal length and radius of curvature?
What is Cartesian sign convention?
Pole is taken as origin.
Incident light travels from left to right.
Distances measured to the right are positive.
Distances measured to the left are negative.
Heights above principal axis are positive.
Heights below principal axis are negative.
State Snell's law.
For two given media, ( n_1 \sin(\theta_i) = n_2 \sin(\theta_r) ) where ( \theta_i ) = angle of incidence, ( \theta_r ) = angle of refraction.
Define refractive index.
The refractive index of a medium is ( n = \frac{c}{v} ) where ( c ) = speed of light in vacuum, ( v ) = speed of light in the medium.
What is absolute refractive index?
The refractive index of a medium with respect to vacuum.
What is relative refractive index?
The refractive index of one medium with respect to another.
What is optical density?
Optical density indicates how much a medium slows down light. A medium with higher refractive index is optically denser.
Why does refraction occur?
Because the speed of light changes when it enters another medium.
What is total internal reflection?
It is the complete reflection of light back into the denser medium when it travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle.
Conditions for total internal reflection.
- Light must travel from denser to rarer medium.
- Angle of incidence must be greater than critical angle.
Define critical angle.
The angle of incidence in the denser medium for which the angle of refraction becomes 90°.
Write the expression for critical angle.
Applications of total internal reflection.
- Optical fibres
- Diamond sparkle
- Prism binoculars
- Endoscopy
What are optical fibres?
Thin transparent fibres made of glass or plastic that transmit light using total internal reflection.
State the lens maker's formula.
State the thin lens formula.
Define power of a lens.
Power is the reciprocal of focal length in metres. Unit = dioptre (D).
What is one dioptre?
The power of a lens whose focal length is 1 metre.
State the prism formula (minimum deviation).
What is angle of deviation?
The angle between the direction of incident ray and emergent ray.
What is minimum deviation?
The least angle of deviation produced by a prism.
What is dispersion of light?
The splitting of white light into its constituent colours when it passes through a prism.
Why does dispersion occur?
Because different colours have different refractive indices in a medium.
Which colour deviates most? Least?
Violet → Maximum deviation, Red → Minimum deviation.
Why is the sky blue?
Because shorter wavelength blue light is scattered more by the atmosphere.
Why are sunsets red?
Because most blue light gets scattered away and mainly red light reaches the observer.
What is a rainbow?
A rainbow is formed due to refraction, dispersion and total internal reflection of sunlight inside water droplets.
Define magnifying power.
Magnifying power is the ratio of the angle subtended by the final image to that subtended by the object at the eye.
What is the least distance of distinct vision?
The minimum distance at which the eye sees objects clearly. For a normal eye, 25 cm.
What is the normal range of vision?
25 cm to infinity.
Why is an objective lens of a microscope of small focal length?
To produce a highly magnified real image.
Why is the objective of an astronomical telescope of large focal length?
To collect more light and obtain high magnification.
Why is the eyepiece of a telescope of small focal length?
To increase magnifying power.
What is accommodation?
The ability of the eye lens to change its focal length so that objects at different distances are seen clearly.
What is the near point?
The closest point from the eye where objects are seen clearly. For a normal eye: 25 cm.
What is the far point?
The farthest point visible clearly without accommodation. For a normal eye: Infinity.
What is myopia?
A defect in which distant objects are not seen clearly. Correction: Concave lens.
What is hypermetropia?
A defect in which nearby objects are not seen clearly. Correction: Convex lens.
What is presbyopia?
An age-related defect due to reduced accommodation. Correction: Bifocal or progressive lenses.
Difference between real and virtual images.
Real image: Can be obtained on screen, formed by actual meeting of rays, usually inverted.
Virtual image: Cannot be obtained on screen, formed by apparent meeting of rays, usually erect.
Why do stars twinkle?
Why does a diamond sparkle?
Why is the sky blue?
Why are sunsets red?
Why is an objective lens of a microscope of small focal length?
Why is the objective lens of a telescope of large aperture?
Why does violet deviate more than red?
Why does a prism disperse white light?
Why is optical fibre based on total internal reflection?
Why is power of a convex lens positive and that of a concave lens negative?