Chapter: Refraction of Light
1. Definition of Refraction:
Refraction is the bending of light when it passes from one transparent medium to another, due to a
change in its speed.
Reason: Light changes speed because different media have different optical densities.
2. Laws of Refraction (Snell's Law):
First Law: The incident ray, the refracted ray, and the normal to the interface all lie in the same
plane.
Second Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is
constant for the given pair of media.
sin
sin
constant
sinr
sini
=constant=μ
This constant is called the Refractive Index (
μ) of the second medium with respect to the first.
3. Refractive Index:
It’s a measure of how much a light ray bends when entering a new medium.
Formula:
Speed of light in vacuum
Speed of light in medium
μ=
Speed of light in medium
Speed of light in vacuum
Higher the refractive index, the slower the light travels in that medium.
4. Factors Affecting Refraction:
Wavelength of Light: Shorter wavelengths (e.g., blue light) refract more than longer wavelengths
(e.g., red light).
Nature of Media: The difference in optical densities between two media determines the amount of
refraction.
5. Refraction through a Rectangular Glass Slab:
When light passes through a rectangular glass slab:
The emergent ray is parallel to the incident ray but is laterally displaced.
The angles of incidence and refraction at the first interface (air to glass) and second interface (glass
to air) are related by Snell’s Law.
6. Refraction through a Prism:
In a prism, light bends twice—once when it enters and once when it exits.
Deviation Angle: The overall change in the direction of light after passing through the prism.
The angle of deviation depends on:
The angle of incidence,
The angle of the prism,
The refractive index of the prism material.
The emergent ray is deviated towards the base of the prism.
7. Total Internal Reflection (TIR):
Definition: When a light ray travelling from a denser to a rarer medium hits the boundary at an angle
greater than the critical angle, it reflects back entirely into the denser medium.
Critical Angle (
C): The angle of incidence in the denser medium for which the angle of refraction in the rarer
medium is 90°.
Condition: TIR occurs only when light travels from a denser medium to a rarer medium.
sin
sinC=
8. Applications of Refraction and TIR:
Lenses: Light bends while passing through lenses, which is used to converge or diverge rays.
Optical Fibers: TIR is used in optical fibers for transmitting light signals over long distances without
loss.
Mirages: A mirage is an optical illusion caused by the refraction of light in layers of air with different
temperatures.
9. Apparent Depth:
An object submerged in water appears closer to the surface than it actually is due to refraction.
Apparent depth formula:
Apparent Depth
Real Depth
Apparent Depth=
Real Depth
10. Refraction in Lenses:
Convex Lens: Converges parallel light rays to a focal point. (Used in magnifying glasses, cameras)
Concave Lens: Diverges light rays; the rays appear to come from a virtual focal point. (Used in
spectacles for correcting myopia)
Important Formulas:
Snell's Law:
sin
sin
μ=
sinr
sini
Refractive Index:
μ=
Where
c = speed of light in vacuum,
v = speed of light in the medium.
Critical Angle:
sin
sinC=
1
Apparent Depth:
Apparent Depth
Real Depth
Apparent Depth=
Real Depth
Key Diagrams:
Ray diagram for refraction through a glass slab.
Ray diagram for refraction through a prism.
Total Internal Reflection in optical fibers.
Quick Revision Points:
Refraction is caused due to the change in speed of light as it passes between media of different
optical densities.
Snell's law helps to calculate the angle of refraction.
Total internal reflection occurs when the angle of incidence is greater than the critical angle, a
principle used in optical fibers.
