Refraction – Snell’s Law (AS and IG)

We know that light is refracted towards the normal when it passes from an optically less to more dense material. Here is a diagram of monochromatic light passing through a glass block. i is the angle of incidence, r is the angle of refraction. Monochromatic means ‘of one colour’. same frequency in other words.

Screen Shot 2.png

The REFRACTIVE INDEX n between two media is the RATIO of the speeds in the 2 materials and is given by

Screen Shot 3.png

We can use either c or v to represent the velocity, but c alone refers to the velocity of light in a vacuum. Blue light (shorter wavelength) has a higher absolute RI than red light

Draw around your Perspex block.

Put in two pins to show the incident ray. Look through the block and line up the object pins with two more on the other side of the block so all four look to be in a straight line.  Take the pins out and draw in the lines. The incident ‘ray’ and the emergent ‘ray’ ought to be parallel.

Draw a normal, measure the angles of incidence and refraction and use the law to find n the refractive index – the ratio of the speeds of light in air and Perspex for white light.


(lab work: Take various values of i and r, plot a graph of sin i against sin r – it should be a straight line through the origin of slope n)

Speed DECREASED in Perspex


Wavelength DECREASED

This link allows you to vary the incident angle. Download and run the simulation (NB – won’t run natively on Android tablet or iPhone – OK on Mac or Windows)

Notice the weak reflected ray and also the wavelength decrease from air to a denser medium – in this case, into water [n=1.33].

Screen Shot 1.png

Clearly if as is the case here the wavelength in material 1 is greater than that in 2, the ray is refracted towards the normal.

Absolute or relative?Screen Shot

Absolute RI is the refractive index of the medium as compared to a vacuum. Relative  refractive index between a pair of media is the ratio of their absolute refractive indices.




Shadows tell us that light travels in straight lines. We can draw a ray (or streak) of light as a straight line on  a piece of paper.

Can you see what’s wrong with this picture?

If only our shadows could tell us what we're thinking...

This is a better shadow picture. Can you see why? When was it taken, in the morning or at night?

A sundial is a clock. Some people still use them in their gardens

It works as long as the dial is pointing the right way to start with...

As the sun passes across the sky during the day, a shadow is cast of a stick in the middle of a dial, which shows the time.