Topic 6

Question 1

Equation: Refractive index *

Answer 1

n = c / v

n = refractive index

c = speed of light in vacuum (3.00x10⁸ ms⁻¹)

v = speed of light in medium (ms^-1)

Question 2

Equation: Path of light ray *

Answer 2

n₁sinθ₁ = n₂sinθ₂

Question 3

What is the refractive index of air?

Answer 3

1.00

Question 4

Equation: Intensity *

Answer 4

I = P / A

I = intensity (Wm²)

P = power (W)

A = area (m²)

Question 5

Definition: Intensity

Answer 5

The energy falling on an area of 1m² per second.

(Wm²)

Question 6

Equation: Critical angle *

Answer 6

sinC = 1 / n

C = critical angle

n = refractive index of material of block

Question 7

What assumptions are made using the critical angle equation?

Answer 7

Light is passing from air into a block of material with a larger refractive index.

Question 8

Definition: Critical angle

Answer 8

The angle at which a ray will travel along the boundary of the block if it enters at this angle.

Question 9

What are the relationships between θ₁ and C?

Answer 9

θ₁ < C

Light passes through block.

θ₁ = C

Light travels along boundary.

θ₁ > C

Total internal reflection.

Question 10

Definition: Focal length

Answer 10

The distance between the focus and lens.

Question 11

Constructing ray diagrams: Converging lens

Answer 11

Question 12

Constructing ray diagrams: Diverging lens

Answer 12

Question 13

Equation: Power of a lens *

Answer 13

P = 1 / f

P = power (D)

f = focal length (m)

Question 14

Equation: Power of multiple lenses *

Answer 14

P = P₁ + P₂ + P₃

Question 15

Equation: The lens formula *

Answer 15

1/f = 1/u + 1/v

f = focal length (m)

u = distance between object and lens (m)

v = distance between image and lens (m)

Question 16

Equation: Magnification *

Answer 16

m = v / u

m = magnitfication

v = image height (m)

u = object height (m)

Question 17

What do you need to do when using lens equations?

Answer 17

For all working, to use negative numbers appropriately.

Question 18

What is a converging lens?

Answer 18

A lens where rays pass through the (RHS) focus and produce a real image.

Question 19

What is a diverging lens?

Answer 19

A lens where rays appear to originate from the (LHS) focus and produce a virtual image.

Question 20

Definition: Diffraction

Answer 20

The spreading out of waves as they pass through gaps or around obstacles.

Question 21

Definition: Huygen’s construction

Answer 21

Every point on a wavefront can be considered as a source of wavelets which spread out in a circle at the same speed. The envelope of all of these wavelets then forms the new wavefront.

Question 22

What are the features of diffraction through a single slit?

Answer 22

As light passes through the slit, it diffracts and where the wavelets of the wavefront are in phase, they interfere constructively, and bright spots are seen.

Question 23

Equation: Diffraction grating *

Answer 23

nλ = dsinθ

n = order of spectrum ‘bright spots’

λ = Wavelength (m)

d = diffraction grating (lines per m)

θ = angle at which order is seen

Question 24

What are the wavelengths of the colours red and blue?

Answer 24

Red = 750 nm

Blue = 450 nm

Question 25

When does maximum diffraction occur?

Answer 25

When the wavelength of light is equal to the size of the gap.

Question 26

What is the relationship between slit seperation and diffraction grating?

Answer 26

Diffraction grating = 1 / slit seperation

Question 27

Equation: Photon enegy *

Answer 27

E = hf

E = energy of photon (J)

h = Planck’s constant (6.63x10^-34 Js)

f = frequency (Hz)

Question 28

Equation: Photoelectric effect *

Answer 28

hf = Φ + ¹/₂mv²

hf = photon energy (J)

Φ = work function (J)

¹/₂mv² = kinetic energy (J)

Question 29

Definition: Work function

Answer 29

The minimum energy needed to free an electron from the surface of a metal.

Question 30

What happens if the photon energy being shone on a metal is lowerthan the work function?

Answer 30

No electrons will be able to escape the metals surface.

Question 31

What are the features of a ‘photoelectric effect’ graph?

Answer 31

Gradient = Planck’s constant

Y-intercept = Work function

X-intercept = Threshold frequency

Question 32

How can the work funtion of a metal be worked out using threshold frequency?

Answer 32

E = hf_o

Question 33

What affect does intensity of light have on the photoelectric effect?

Answer 33

• Number of electrons emitted per second increases:

The higher the intensity, the more electrons are emitted.

• E_k of electrons remains the same:

Frequency of photons arriving at metal is unchanged, so E_k doesn’t change.

Question 34

Definition: Photon

Answer 34

A discrete quantity of light energy.

Question 35

What assumption is made when using the photon energy equation E = hf?

Answer 35

Energy is in joules.

Question 36

How does the photoelectric effect cause something to become charged?

Answer 36

1. A photon is absorbed by the object, and if it has a frequency higher than the threshold frequency, the work function of the metal will be exceeded.
2. Therefore, electrons will be emitted.
3. This mean the object will become more positivey charged.

Question 37

Definition: Refracton

Answer 37

When a wave changes speed as it moves from one medium to another causing it to change direction.

Question 38

Definition: Photoelectric effect

Answer 38

The emission of electrons from a metal when illuminated by light.

Question 39

Definition: Excitation

Answer 39

Electrons gain energy and move to a higher energy level.

Question 40

Definition: De-excitation

Answer 40

Excited electrons move to a lower energy level by emitting photons.

Question 41

Why are only certain frequencies of photons emitted or absorbed?

Answer 41

Atoms have discrete energy levels which electrons are able to move between. Therefore, the electrons can only gain or lose a specific amount of energy, in the form of photons. Because E = hf, these photons can only have certain frequencies.

Question 42

Definition: Energy level

Answer 42

The discrete amount of energy an electron can have.

Question 43

How is an absorption spectrum produced?

Answer 43

1. An electron moves to a higher energy level of discrete value by absorbing a photon.
2. Therefore, only photons with a specific amount of energy are absorbed which match the difference in energy between two energy levels.
3. This means these photons cannot be seen on the spectra, so a black line forms.

Question 44

How is a line spectrum produced?

Answer 44

1. Electrons gain energy and move to a higher discrete energy level.
2. The electron then falls back down to a lower discrete energy level by emitting a photon.
3. This photon has a specific amount of energy, and therefore a specific frequency.
4. The photons released produce a line spectrum.

Question 45

What is total internal reflection?

Answer 45

When all of the light is reflected due to the angle of incidencebeing greater than the critical angle.

Question 46

Why are electrons useful for investigating the atomic structure of materials?

Answer 46

They have a very small wavelength, similar to the size of the atom.

Question 47

Definition: de Broglie wavelength

Answer 47

The wavelength associated with a particle due to its momentum.

Question 48

What does the pattern produced when directing a beam of electrons onto graphite tell us about the behaviour of electrons?

Answer 48

An interference pattern is produced suggesting the electrons behave as waves. If you were to increase the speed of the electrons, they would have more momentum and therefore the circles of the pattern would be smaller.