Definition Questions

Question 1

What is a spectral line?

Answer 1

When light is split up l, each photon energy frequency is seen as a separate discrete line of colour

Question 2

Why do atoms of different elements produce different colours?

Answer 2

Different atoms have different discrete energy levels.

The photon energy depends on the difference between these energy levels.

Different colours correspond to photons of different energies

Question 3

Explanation of how electrons can escape from the surface of a cathode

Answer 3

Electrons near the surface of the cathode absorb photon and gains energy.

The work function is the energy needed for the electron to escape the surface.

Electrons released this way are called photoelectrons.

Question 4

Explanation of why photocathodes have a threshold wavelength

Answer 4

Electrons require a certain amount of energy to escape from the surface.

This energy comes from a photon and so the photon needs a minimum frequency to provide enough energy.

Minimum frequency corresponds to the threshold wavelength

Question 5

Explanation of why some photoelectrons will be emitted with less than the maximum kinetic energy

Answer 5

Photon transferred to an electron below the surface

So some energy transferred to atoms on way to the surface so will leave with less than max

OR: lower energy photon could provide electron with just under max

Question 6

The absorption and emission of a narrow band of frequencies by electrons in some liquids

Answer 6

When blue light (for example) is absorbed and the electron is excited to a higher energy level

As the electron loses its energy it emits green light (for example) and falls to the intermediate energy level

Due to liquids having close together particles, there are bands rather than discrete levels

Hence a range of frequencies

Question 7

How plane polarised light is different from ordinary light

Answer 7

Plane polarised light oscillates in 1 plane.

Ordinary light oscillates in all planes

Question 8

What effect does a polarising filter have on polarised light

Answer 8

Only allows light in 1 plane.

When filter is at 90° to light, blocks it out

Question 9

A method of measuring the angle of rotation of a plane of polarisation

Answer 9

Polaroid filters placed at both ends of a tube.

Polaroids crossed to block out light, with no sugar solution in between.

Solution introduced

One Polaroid rotated, angle measured

Question 10

What is a transverse wave?

Answer 10

Oscillations occur at right angles to the direction of energy transfer

Question 11

What is a longitudinal wave?

Answer 11

Oscillations occur parallel to the direction of energy transfer

Question 12

What is frequency?

Answer 12

The number of oscillations per unit of time

Question 13

What is fundamental frequency?

Answer 13

The longest wavelength standing wave that fits in the wire (in the column of air)

Question 14

What is wavelength?

Answer 14

The distance between successive points in phase

Question 15

What is superposition?

Answer 15

When two or more waves meet, the resultant wave has a displacement equal to the sum of the two

Question 16

How is a standing wave formed?

Answer 16

Waves are reflected from a barrier.

The incident and reflected waves superpose.

Where they meet in phase, constructive interference occurs and forms antinodes

Where they meet in antiphase they form a displacement of zero, nodes

Question 17

What is coherence?

Answer 17

Same frequency and a constant phase relationship

Question 18

Why does the resistance of a metal increase with temperature?

Answer 18

Increased temp leads to increased lattice vibrations

Causes electrons to collide and not get through as easily (increased resistance)

Question 19

Why does the resistance of a semiconductor decrease with temperature?

Answer 19

Increased temp leads to more electrons released

This increases current and lowers resistance

Dominates effect of lattice vibrations

Question 20

What effect does internal resistance have on power?

Answer 20

Power has a maximum value when external resistance is = to internal resistance

Question 21

Why does the p.d across a load resistor increase as current falls?

Answer 21

Load resistor is in series with internal resistance

Current falls, load resistor has a greater share of current than the internal resistance

Question 22

Explanation of how ultrasound can be used to build up a picture

Answer 22

Reflects from boundaries of different densities.

Time taken to reflect gives depth

Size of reflection gives density

Question 23

Definition of Doppler shift

Answer 23

A change in frequency caused by the relative moment between source and receiver

Question 24

Principal of operation of a Doppler probe

Answer 24

Arrange probe so fluid is approaching

Fluid reflects

Change in frequency depends on speed

Question 25

Diffraction

Answer 25

Spreading out of a wave as it passes through a gap or object

This effect is greatest when the object or gap is a similar size to wavelength

Question 26

How do atoms emit light?

Answer 26

Electrons are excited to higher energy levels, as they fall they emit this energy in the form of photons