O Level Notes : Physics - Light (Topic 2)

Refraction of Light:

Coming to refraction, it is the bending of light when it enters another medium. For example, if light enters from air to water, it will bend. This happens due to the change in speed of light as it changes medium. That change of speed causes bending. This phenomenon is called refraction.

To summarize:

• Refraction is caused by the change in speed of light.
• At the boundary of 2 optical media, if there is a sudden change in the speed of light, it will cause the path of light to bend.
• Light travels fastest in air/vacuum.

Laws of Refraction:

• The incident ray, the refracted ray and the normal lie in the same plane.
• For two particular media, the raio of sine of incident angle to the sine of refracted angle is a constant.

i.e. sin i / sin r = constant

Refractive Index:          

The refractive index of a medium is a ratio between the speed of light in air and the speed of light in a medium. Therefore,

Speed of light in air / Speed of light in medium = n

Since refractive index is a constant, we can conclude

sin i / sin r = refractive index

where i is the speed of light in air/ vacuum and r is the speed of light in any medium. (Note the position of numerator and the denominator: the denominator always has the denser medium.)

The greater the value of refractive index of a medium, the greater the bending of light, and the more denser the material is.   

When the ray of light travels from a less dense medium to a denser medium, the ray of light bends towards the normal because the speed of light decreases. And vice versa. This is something which should be on your fingertips!

In this image, the ray of light is bending towards the normal as it enters from air to glass because glass is a denser medium. It’s not that difficult, just remember that the denser medium is the slower medium. And the medium in which light travels slower, it will eventually bend towards normal.

Another important thing is that when light enters a different medium, no matter if its denser or lighter, the light ray will not bend at all. It will pass straight through.

Total Internal Reflection:

Total internal reflection occurs when light travels from an optically denser medium to a less dense medium. For example, when travelling from glass to air.

Critical Angle is the angle of incidence in the optically denser medium where the angle of refraction in the less dense medium = 90°.

Take a look at this diagram, it will make it clearer:

Now to further clear it out, understand the cases below:

In first case, most of the light is refracted while some of it is reflected back when the angle of incidence is less than the critical angle.

In the second case, the angle of incidence is equal to the critical angle. Most of the light travels along the surface because it is refracted at 90° angle.

Now in the third case, the angle of incidence is further increased such that it is greater than the critical angle. Now reflection, instead of refraction will take place. All the light will be totally reflected in the same medium. This is called total internal reflection.

Task: Google the applications of total internal reflection to find out where it is used.

Thin Converging Lenses:

A lens is a clear plastic or glass with curved surface.

Main features of a converging lens:

• Optical Centre (C): The midway point between the lens surface on the principal axis
• Principal axis: The line passing symmetrically through the optical centre of the lens
• Principal focus (F): Point on the principal axis where rays of light converge after passing through the lens
• Focal length (f): Distance between the optical centre, C and the principal focus F.
• Focal plane: Plane which passes through F and P. It is perpendicular to principal axis.

Okay the line passing through the centre of the lens is the principal axis. Here are certain rules which we have to follow while constructing a ray diagram:

• (The blue line in the image above) Any ray passing through the centre of the lens will not bend and will pass straight through.
• (The red line in the image above) Any ray passing parallel to the principal axis will bend and pass through the principal focus.
• (The green ray in the image above) Any ray passing through the principal focus will bend parallel to the principal axis.