REFLECTION AND REFRACTION (Lecture1)

Optics

Deals with light & other waves behaviour.

Wave front

Any spherical surface that is concentric with the source.

Sound waves vs light waves

Sound waves spread out from a source & light waves spread out from an emitter.

Rays

Paths of particles drawn as an imaginary line along the direct line of travel of the wave.

Rays of waves in uniform isotropic material

Rays are straight lines normal to wave fronts.

Uniform isotropic material

Material with same properties in all regions and direction.

Boundary surface between 2 materials can change

Wave speed & ray direction.

Rays of spherical wave fronts

Are radii.

Wave segments in air and glass

Are straight lines.

Geometric optics

Optics branch where ray description is adequate.

Physical/ wave optics

Optics branch dealing with wave behaviour.

When a light wave strikes a smooth interface separating 2 transparent materials

Wave is partly reflected and partly refracted (transmitted).

Ray beam composition.

Composed of bundles of rays.

Specular reflection

Reflection at a definite angle from a very smooth surface.

Diffuse reflection

Scattered reflection from a rough surface

Transmission and reflection on rough interface

Both transmitted and reflected light are scattered in various directions as there is no single angle of transmission/ reflection.

Plane of incidence

Where incident, reflected, refracted rays and the normal to the surface lie.

Law of reflection

angle of reflection is equal to the angle of incidence.

Fermat’s principle (principle of least time)

Path taken between 2 points by light ray is the path that can be traversed in the least time.

Source/ object

Anything from which light rays radiate.

Self-luminous (e.g. lightbulb filament)

Light emitted by the object itself.

Point object

Has no physical extent.

Extended objects

Real objects with length, width and height made up of lots of point objects.

Creation of image point from light rays

Light rays coming from a point on an object are deflected by reflection, refraction, or both, so they converge towards or appear to diverge from a point called an image point.

Image distance

Distance of image point from mirror.

Relationship between object & image points.

Are equidistant from mirror & are perpendicular to mirror when connected.

Virtual image

Outgoing rays don’t pass through the image point.

Absorption

Light intensity decreases when it travels through any material (except vacuum).

Refraction

Direction change of wave when it crosses a surface between materials with different optical properties.

Snell’s Law describes

Direction changes of ray travelling through the interface between 2 materials.

Index of refraction (n)

Ratio of light speed in vacuum to the speed in the material.

Speed of light in vacuum ( c )

299,792,458 m/s

Speed of light (v)

Decreases in material.

Internal reflection

Light is incident at an angle which would need sin(theta) > 1.

If angle is theta = 90 degrees (sin(theta) = 1)

Refracted ray travels along the surface.

Fibre optics

Flexible fibres that can be bundled in thousands, each 0.01mm in diameter.

Condition allowing light to undergo total internal reflection in transparent rod

Rod's index of refraction is greater than that of the surrounding material.

Trapping light in curved rod

Light can be trapped within the rod through total internal reflection, allowing it to remain within the rod even if the rod is slightly curved.

Transmitting image in transparent rod or fibre bundle

Can transmit an image if the fibres are assembled in bundles, maintaining the same relative positions at both ends, resulting in mirror images.

Common medical application of fibre optics that uses transparent rods or fibre bundles

Endoscopes, which are inserted directly into organs such as the bronchial tubes, bladder, colon, blood vessels, and other areas for direct visual examination.