Light and Sound

Question 1

What type of wave are light waves?

Answer 1

Transverse waves (as they are part of the EM spectrum)

Question 2

What is reflection?

Answer 2

When a wave hits a boundary between two different media (materials), but does not pass through and instead is sent back, staying in the same medium

For sound waves this is an echo. For light waves, it is when you see a reflection such as a mirror

Question 3

What is refraction?

Answer 3

When a wave passes between two different media (materials) of different densities and undergoes a change in direction

If light is being refracted, the two different media must both be transparent

Question 4

What is the law of reflection?

Answer 4

• When a wave is reflected, the angle between the incident ray (original ray) and normal, and the reflected ray and the normal will be equal
• So angle of incidence = angle of reflection

The normal is a straight line drawn through the middle where the two points meet at the boundary, and is perpendicular to the boundaries

Question 5

How do you draw a reflection ray diagram?

Answer 5

• The normal should be drawn
• The angle of incidence and the angle of reflection must be drawn accurately, be labeled i and r and must be equal

Question 6

What happens to light when it enters a more dense material?

Answer 6

• The wavespeed and the wavelength decrease
• Light bends towards the normal

The frequency here is staying constant meaning colour does not change and wavespeed and wavelength are directly proportional

Question 7

What happens to light when it enters a less dense material?

Answer 7

• Its wavespeed and wavelength will increase
• Light bends away from the normal

Question 8

What happens to light if it is passing into a different material but is doing so perpendicular to the boundary (it is going along the normal)?

Answer 8

It does not change direction at all

Question 9

How do you draw a refraction ray diagram?

Answer 9

• Draw in the normals, they must be perpendicular to the surface of the boundary
• As the light passes into a more dense material, the light will keep going but bend towards the normal - The angle of incidence and the angle of refraction (in) will be formed
• As it passes out of the dense material, the light will keep going but bend away from the normal, and the angle of emergence and the angle of refraction (out) will be formed

The angles are always between the normal and the waveline

Question 10

What happens to the angle of refraction (in) as the angle of incidence increases?

Answer 10

The angle of refraction increases

Question 11

What is the equation which links the angle of incidence, angle of refraction and the refractive index of the material the waves entering?

The refractive index is a number related to the speed of light within the material, so is related to the density. Objects with higher densities will have a higher refractive index (diamond has 2.4)

Answer 11

• Refractive index = Sin (angle of incidence)/Sin (angle of refraction - in)
• N = Sin (i)/Sin (r)

Question 12

What is total internal reflection?

Answer 12

When light passes from a dense to a less dense material, but instead of being refracted, it is all reflected

Question 13

What are the conditions for total internal reflection to occur?

Answer 13

• The angle of angle of incidence must be greater than critical angle (c)
• It must be passing from a more dense to a less dense medium

The law of reflection still applies so the angle of incidence must equal the angle of reflection during TIR

Question 14

What is critical angle (c)?

Answer 14

When the angle of emergence/refraction (in) is 90 degrees or greater and fails to leave the medium in which it entered, the angle of incidence has reached or exceeded critical angle (c)

Question 15

How is total internal reflection used in optical fibres?

Answer 15

• Light is sent through the fibre, and each time it hits the wall of the fibre it is totally internally reflected and continues to travel down the fibre, even if there is a bend
• This means it can be used to see inside peoples bodies and transmit information

Question 16

How is total internal reflection used in prisms?

Answer 16

• Light hits the right-angled prism, and totally internally reflects and travels perpendicular to the direction it was originally travelling in
• This is used in periscopes

Question 17

What is the relationship between critical angle (c) and refractive index?

Answer 17

Sin(Critical angle (c) = 1/Refractive index
Sin(c) = 1/n

Question 18

What type of wave is a sound wave?

Answer 18

Longitudinal

As sound is a wave, it can also be reflected and refracted

Question 19

What is the speed of sound in air?

Assume standard temperatures and altitudes

Answer 19

340m/s (760 miles per hour)

Question 20

What must you take into account when measuring the speed of sound using an echo?

Answer 20

The time it takes to travel to the wall and back

Question 21

How are oscilloscopes used to measure and display sound waves?

Answer 21

They display the waves as if they are transverse waves, with time periods, frequency, amplitudes etc.

Question 22

What determines the pitch of a sound?

Answer 22

• The frequency of the waves
• The higher the frequency, the higher the pitch

Question 23

What the determines the loudness of a sound?

Answer 23

• The amplitude of the waves
• The higher the amplitude, the louder the sound

Question 24

What is the range of human hearing?

Answer 24

20Hz-20000Hz

Hz is a measure of frequency, so a measure of pitch