Stars twinkle because light from stars is refracted as it passes through Earth's atmosphere, causing the star to appear to shift positions. Refraction is the redirection of light when passing between two different mediums, such as from space into the atmosphere. Snell's law describes the relationship between the angles of incidence and refraction based on the refractive indices of the two mediums the light is passing through.

1. Important Terms

2. Why do starts twinkle?
• Star as seen from space
• Light deviates from its path as it travels through
various layers of atmosphere
• Star as observed from earth.

3. Refraction
• Refraction is the redirection of a wave (light) as
it passes from one medium to another.

4. Laws of Refraction
• Snell’s law
• “The ratio of the sine of the angle of incidence to the sine of the angle of
refraction is a constant, for the light of a given colour and for the given pair
of media”.
• The incident ray, reflected ray and the normal, to the interface of any
two given mediums; all lie in the same plane.

5. Snell’s law
• N1 = refractive index of medium 1
• N2 = refractive index of medium 1
• θ1 = angle of incidence
• θ2 = angle of refraction
n =
𝑐
𝑣
=
𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑣𝑎𝑐𝑢𝑢𝑚
𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚