Refraction

Refraction

Refraction is the bending of light as it travels from one medium to another; it occurs when the speed of light changes at the boundary.

Laws of Refraction

Incident ray, normal, and refracted ray lie in the same plane.

Snell’s Law: sin i / sin r = constant.

Direction of Refraction

From rarer → denser medium, light bends towards the normal; from denser → rarer medium, it bends away.

Refractive Index & Speed

Higher refractive index means slower light speed in the medium.

Virtual Image & Apparent Depth

A virtual image is where rays appear to diverge. Apparent depth is the shallower position of this image due to refraction.

Critical Angle

The critical angle is the incidence angle in a denser → rarer medium that produces a 90° refracted ray.

Conditions for Total Internal Reflection

Total internal reflection requires light to travel from denser → rarer medium and incidence angle > critical angle.

Optical Fibre Principle

Optical fibres transmit light by repeated total internal reflection inside the core.

Optical Fibre Coating

Fibres are coated with lower-index material to prevent leakage and keep total internal reflection.

Prisms & Reflection

A 45°–90°–45° prism can turn light 90° (single reflection) or 180° (double reflection).

🔹 Glossary
Angle of incidence (i)

Angle between incident ray and the normal.

Angle of refraction (r)

Angle between refracted ray and the normal.

Apparent depth

Depth an object appears to be due to refraction (virtual image).

Critical angle (C)

Incidence angle in a denser → rarer medium where refraction is 90°.

Incident ray

Ray of light that strikes a surface.

Normal

Line drawn at 90° to the surface at the point of incidence.

Optical density

How much a medium slows or bends light.

Optical fibre

Thin transparent fibre transmitting light via total internal reflection.

Real depth

The actual depth of an object submerged in a medium.

Refracted ray

Light ray that has entered a new medium and bent.

Refraction

The bending of light when moving between transparent media.

Refractive index (n)

Ratio sin i / sin r (constant for a medium).

Snell’s Law

Law of refraction: sin i / sin r = constant.

Snell’s Window

Cone of light visible to an underwater observer looking up.

Total internal reflection

Light fully reflects inside a denser medium when incidence > critical angle.

Transparent medium

Substance that allows light to pass through.

Virtual image

Image formed where rays appear to diverge but don’t actually meet.