Reflection and Refraction Elliott

Basics Light Travels in straight lines at a speed of 3 × 108 m/s in air.  When light hits an object it is: Reflected Transmitted  Absorbed.

: Reflection All reflection depends on the smoothness of the surface. If the surface is smooth, then parallel rays are reflected parallel (specular reflection). If the surface is rough, then parallel rays are scattered.

Mirrors The image in a mirror has these features: It is the same distance behind the mirror line as the object is in front. It is the same size as the object.  It is the right way up (erect).  It is laterally inverted, i.e. left and right swapped over.  It is virtual, meaning that the rays cannot be projected onto a screen.  The image is not really behind the mirror; it just appears to be.

Absolute Refractive Index We can also describe the absolute refractive index as the ratio of the speed of light in a vacuum to the speed of light in an optical medium.   (n for a vacuum = 1)

Absolute Refractive Index The refractive index given the physics code n and has no units.   Glass has a refractive index of 1.5, so the speed of light is: 3.0 × 108 m s-1 ÷ 1.5 = 2.0 × 108 m s-1.

Relative Refractive Index

Points of Note Absolute refractive index is the ratio between the speed of light in a vacuum and speed of light in a material. It is always greater than 1.0, as there is no material optically less dense than a vacuum. The relative refractive index is the ratio between the speed of light in material 1 and the speed of light in material 2. Relative refractive index can be less than 1. There are no units for refractive index as it's m s-1 / m s-1.

Check Your Progress What is the speed of light in glass, refractive index 1.5, when the speed of light in air is 3.0 x 108 m s-1?

Answer n2/n1 = c1/c2 1.5 ÷ 1.0 = 3.0 × 108 m s-1 ÷ c2 c2 = 3.0 × 108 m s-1 ÷ 1.5 = 2.0 × 108 m s-1

Snell’s Law Quantitative relationship between angle of incidence and angle of refraction.

The ratio n2/n1 is the relative refractive index, which we can also define as the ratio of the sines of the angles of incidence and refraction:

Notation The ratio n2/n1 can also be written as as 1n2. This means the refractive index of light going in from material 1 to 2.

Check Your Progress What does 2n1 mean? 2. A ray of light strikes an air-glass boundary at an incident angle of 30o.  If the refractive index of glass is 1.5, what is the angle of refraction?

Answer Going from material 2 to material 1. 2. 1n2 = sin q1/sinq2 1.50 = sin 30 ÷ sin q2  = 0.500 ÷ sin q2 sin q2 = 0.500 ÷ 1.5 = 0.333 q2 = sin-1 0.333 = 19.5 o