Refraction of Light

When light passes obliquely from one transparent medium to another, its speed changes. Due to this change in speed, the light ray bends at the boundary between the two media. This bending of light is called refraction of light.

Refraction of Light

Light bends when it passes from one medium to another at an angle.

Refraction

Incident Ray: The ray that falls on the boundary between two media.

Refracted Ray: The ray that enters the second medium after bending.

Normal: A line drawn perpendicular to the boundary at the point of incidence.

Angle of Incidence: The angle between the incident ray and the normal.

Angle of Refraction: The angle between the refracted ray and the normal.

Rarer Medium: A medium in which light travels faster is called an optically rarer medium.

Denser Medium: A medium in which light travels slower is called an optically denser medium.

Rarer to Denser Medium

Light bends towards the normal.

Denser to Rarer Medium

Light bends away from the normal.

Laws of Refraction

👉 The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane.

👉 For a given pair of media and a given colour of light, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.

This is called Snell's law.

Snell's Law

For a given pair of media, sin i / sin r remains constant.

For light going from medium a to medium b:

Here, is the refractive index of medium b with respect to medium a.

Refractive Index and Speed of Light

The refractive index of a medium tells us how much the medium slows down light.

If the speed of light in vacuum is c and the speed of light in a medium is v, then the absolute refractive index is:

Since light travels slower in any material medium than in vacuum, the refractive index of a material medium is generally greater than 1.

For two media a and b:

Here, v_a is the speed of light in medium a, and v_b is the speed of light in medium b.

Cauchy's Formula

The refractive index of a transparent medium depends on the wavelength of light. According to Cauchy's formula:

For school-level use, we often write the simpler form:

Here, A, B and C are constants for the medium, and is the wavelength of light.

Examples of Refraction in Daily Life

⭐ A pencil partly dipped in water appears bent.

⭐ The bottom of a swimming pool appears raised.

⭐ Stars appear to twinkle due to atmospheric refraction.

⭐ A coin inside water appears to be at a higher position.

⭐ A lens forms images by refraction of light.

Path of Light in Rarer and Denser Media

Important Points to Remember

⭐ Refraction is caused by change in speed of light.

⭐ Light bends towards the normal when it goes from a rarer medium to a denser medium.

⭐ Light bends away from the normal when it goes from a denser medium to a rarer medium.

⭐ The frequency of light does not change during refraction.

⭐ Snell's law gives as a constant for a given pair of media.

⭐ Refractive index has no unit.

Common Mistakes

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