1 Introduction
Light is a form of energy that enables us to see objects. When light travels from one medium to another, its speed changes. Due to this change in speed, the direction of light also changes.
Refraction of light is the bending of light when it passes from one transparent medium to another.
2 Refraction of Light
Refraction of light is the change in direction of light when it passes obliquely from one transparent medium to another.
Refraction occurs because the speed of light is different in different media such as air, water, glass, etc.
Key Point: Refraction only occurs when light passes obliquely (at an angle) from one medium to another. If light enters perpendicularly, it passes straight without bending.
3 Causes of Refraction
Refraction occurs due to:
- Change in speed of light in different media
- Difference in optical density of the two media
Light travels slower in optically denser media and faster in optically rarer media.
4 Laws of Refraction (Snell's Laws)
The incident ray, refracted ray, and the normal at the point of incidence all lie in the same plane.
For a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.
This constant is called the refractive index of the second medium with respect to the first medium.
5 Refractive Index
The refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium.
Where:
- n = refractive index
- c = speed of light in vacuum (3 × 10⁸ m/s)
- v = speed of light in the medium
A higher refractive index means the medium is optically denser.
6 Absolute and Relative Refractive Index
Absolute refractive index: Refractive index of a medium with respect to vacuum.
Relative refractive index: Refractive index of one medium with respect to another.
7 Refraction Through a Rectangular Glass Slab
When a ray of light passes through a rectangular glass slab:
- The ray bends towards the normal on entering the slab
- The ray bends away from the normal on leaving the slab
- The emergent ray is parallel to the incident ray
- There is a lateral displacement of the ray
8 Lateral Displacement
The lateral displacement is the perpendicular distance between the incident ray and the emergent ray after passing through a glass slab.
It depends on:
- Thickness of the slab
- Angle of incidence
- Refractive index of the slab
9 Real and Apparent Depth
When an object is placed in a denser medium like water, it appears raised when seen from a rarer medium like air. This is due to refraction.
Important: Apparent depth is less than the real depth. The relationship is: Apparent depth = Real depth / Refractive index
10 Refraction Through a Prism
A prism is a transparent object with two triangular ends and three rectangular sides.
When light passes through a prism:
- It bends twice
- The emergent ray deviates from its original path
11 Dispersion of Light
Dispersion of light is the splitting of white light into its constituent colours when it passes through a prism.
The sequence of colours obtained is: VIBGYOR
Violet deviates the most and red deviates the least due to their different wavelengths.
12 Applications of Refraction
- Formation of images in lenses
- Working of spectacles
- Apparent bending of a stick in water
- Optical instruments like microscope and telescope
- Mirage formation
- Twinkling of stars
Important Points for Examination
- Always draw neat ray diagrams with proper labeling
- Write laws of refraction clearly with proper statements
- Use correct symbols and formulas with units
- Explain answers with proper reasoning and diagrams
- Practice numerical problems on refractive index
- Remember the VIBGYOR sequence for dispersion