Waves & Optics- Reflection and Refraction

Question 1

Reflection and Refraction

Answer 1

• Reflection occurs when a wave hits a boundary between two media where the wave speeds differ but the wave stays in the original medium instead of passing into the second medium The angle of incidence equals the angle of reflection
• Refraction is the change of the propagation direction of waves when they pass into a medium where they have a different speed. It happens whenever the waves are incident to the surface at an angle different to the normal to the surface. When moving to a medium where the waves slow down, the wave direction moves towards the normal (Snell’s Law).

Question 2

Incidence and Reflection

Answer 2

• When a wave hits a flat surface, the angle between the incident direction and the normal is the same as the angle between the reflected direction and the normal.
• While the wave direction changes, the wave speed, wavelength and frequency remain the same.

Question 3

Refractive Index

Answer 3

• Refractive index is the ratio of the speed of light in a vacuum to the speed of light in the medium
• Dimensionless quantity and used to describe how much a light beam is refracted when it passes between media with different refractive indices
• Also determines how much light is reflected at the interference and the value of the critical angle for total internal reflection
• refractive index of a material may be different for different wavelengths of light
• This can result in different colours being split up as light passes through a glass block.

Question 4

Snell’s Law

Answer 4

• Refraction is described by Snell’s law, which relates the angle of incidence and the angle of refraction to the refractive index of the two media.
• Upon refraction, the speed of light changes.
• the time between two wavefronts reaching the surface in the first medium is the same as the time between two wavefronts leaving the surface in the second medium

Question 5

Total Internal Reflection

Answer 5

• In most cases, a fraction of the light incident on the surface between two media is refracted, while the rest is reflected in the first medium. This is called internal reflection.
• For light passes from a medium with a higher refractive index to one with a lower refractive index, there is a critical angle of incidence above which all light is reflected. The phenomenon is called total internal reflection.
• The value of the critical angle can be determined from Snell’s law.

Question 6

Reflection from a curved surface

Answer 6

• Light can also be reflected off curved surfaces
• In this case the new direction of the reflected beam will depend on the particular part of the mirror that the incoming ray hit.
• The light beam will behave as if it hit a flat mirror which is a tangent to the curve of the mirror.
• Parallel beams hitting this surface will all have different angles of incidence but the angle of reflection must be equal to their angle of incidence.

Question 7

Phase Change on Reflection

Answer 7

• When a wave is reflected off the boundary with a medium with a higher refractive index, the wave undergoes a change in phase of π rad on reflection.
• When a wave is reflected off the boundary with a medium with a lower refractive index, the wave is reflected with no phase change.
• This phenomenon is important in thin-film interference, for example, in a soap bubble where the colour is produced by light reflecting off the bubble at the two different interferences.
• The phase change is important in determining which wavelengths of light interfere constructively and destructively and which colours you see at different angles.