Refraction and Snell's Law

What is refraction?

The change in direction of a wave as it passes from one medium to another

When does refraction occur?

1. When a wave goes from one medium to another at an angle to the normal, refraction occurs.

2. This is also due to the change in speed of the wave

Explain why a car, coming at an angle from the road to a muddy track, drifts and changes direction

1. The front, left wheel, entering the muddy track first, is travelling at a different speed compared to the remaining 3 wheels which are travelling at the same speed.

2. This causes the car to change direction, travelling towards the normal because the car is travelling from a less dense to a dense medium.

What is the word equation for the refractive index of a substance?

Refractive index= speed of light in a vacuum // speed of light in the substance.

What is the symbol equation for the refractive index of a substance?

n = c / v or n = c / cs

If the first medium is air, what is the equation for Snell's Law

1. n = sinθ1 / sinθ2

2. θ1 is the angle of incidence

3. θ2 is the angle of refraction

If the first medium is not air, what is the equation for Snell's Law? And explain what happens when the angle of incidence = the critical angle

1. n1sinθ1 = n2sinθ2

2. When the angle of incidence(θ1) is the same as the critical angle, the angle of refraction(θ2) is equal to 90 degrees

3. n1 is the refractive index of the first material

4. n2 is the refractive index of the second material

What is the critical angle in glass?

1. The critical angle is the angle at which the refracted light ray will travel between the boundary of the two media at an angle of 90 degrees

2. This angle is 42*

Explain what happens when a light wave travels from air to glass

1. The light wave will refract towards the normal because the wave is travelling from a less dense to a dense medium

2. Part of the wavefront begins to slow down as it enters the dense medium, whilst the other part will travel at the same,faster speed. Due to part of the wavefront entering the medium at a different time and travelling at a different speed, the light changes direction.

3. As the light wave leaves the glass(at the same angle that the wave entered the medium), the wave will refract away from the normal as the wave is travelling from a dense to a less dense medium.

4. In this case, part of the wavefront begins to speed up as it leaves the medium, whilst the other part is travelling at a same, slower speed. Because part of the wavefront leaves the medium at a different time and faster speed to the other part, this stimulates the change in direction of light.

5. However, light will only leave the medium if the angle inside the medium is less than the critical angle of glass(42*). If the angle of incidence inside the glass is greater than the critical angle of glass, total internal reflection would occur.

Explain total internal reflection

1. Total internal reflection occurs when the angle of incidence inside the glass is greater than the critical angle of glass(42*).

2. This is when no light leaves the medium but rather when whole wave is reflected internally.

Please derive the equation for the critical angle and explain how you have achieved this.

1. Based on Snell's Law n1sinθ1 = n2sinθ2.

2. In this scenario, we are given that the angle of incidence is the same as the critical angle(sinc). Therefore, n1sinc = n2sinθ2. We rewrite this equation to make the critical angle the subject

3. sinc = n1 // n2sinθ2

4. When the angle of incidence is equal to the critical angle, the ray of light will travel through the boundary between the two media, at an angle of 90*; therefore, the angle of refraction(i.e., θ2) will be 90*.

5. sinc = n1 // n2 x sin90

6. Given that sin90 = 1, sinc = n1 / n2 x 1, which is the same as sinc = n1 / n2

What is the equation for the critical angle?

1. sinc = n2 / n1

2. n1 is the refractive index of the first material

3. n2 is the refractive index of the second material

Given that the refractive index of air is 1, rewrite the equation for the critical angle

sinc = 1 / n

Explain what happens to the wavelength of a wave as it passes from air to glass.

1. The speed of the wave will decrease because the wave travels from a less dense to a dense medium.

2. This will cause the wavelength to decrease as v = fλ, with the frequency remaining constant during refraction.

Explain what happens to a ray of light as it passes from a less dense to a dense medium in terms of refractive index

1. As a light wave passes from a medium of a lower refractive index to a medium of a higher refractive index, the light wave will bend(refract) towards the normal.

2. This is the case when n2 is greater than n1

Explain what happens to a ray of light as it passes from a dense to a less dense medium in terms of refractive index

1. As a a light wave passes from a medium of higher refractive index to a medium of lower refractive index, the light wave will bend(refract) away from the normal

2. This is when n1 is greater than n2

What is the angle of incidence?

The angle between the light ray and the normal

What is the definition of the refractive index?

The refractive index is the ratio of the speed of light in a vacuum and the speed of light in the medium

What is the speed of light?

3 x 10^8 m/s

What is the refractive index of air?

1.0

What is the refractive index of glass?

1.5

Why does the refractive index have no units?

1. The refractive index is the ratio of the speed of light in a vacuum to the speed of light in the medium.

2. Therefore, as the units for speed is ms-2. ms-2 // ms-2 is equal to 1.

Explain the dispersion of white light into a spectrum of colours as a light wave passes through a prism.

1. White light is comprised of a continous spectrum of colours, where each colour has a different wavelength (and frequency).

2. The refractive index(n) is different for each colour, with the colours that have a shorter wavelengths(violet light) have a higher refractive index, whilst the colours with the longer wavelengths(red light) have a lower refractive index.

3 Because n = c / v, the wavelengths travel at different speeds in glass and refract(bend) at different amounts at the boundaries. For instance, violet light slows down the most and refracts the most, whilst red light slows down the least and refracts the least.