KCL Section 6: The Photoelectric effect (Waves and photons)

Question 1

What does intensity measure?

Answer 1

Intensity measures how much energy reaches an area per second.

Question 2

An advertisement for a green laser pointer gives the data: 
Wavelength 532nm 
Power 5 mW
Beam diameter at source  1.5 mm
Current 300 mA
Power supply 2 x 1.5V AAA cells

1) What is the efficiency of the laser in converting electrical energy to light energy?
2) What is the intensity (radiation flux density) of the light beam close to the laser?
3) How much energy is in one photon of the light?
4) How many photons per second are emitted by the laser?
5) The manufacturer claims that this laser is ‘much brighter than a 5 mW red laser’. Suggest why this statement may be justified.

Answer 2

1) 0.5%
2) 2.8
3) 3.7 x 10^-19
4) 1.4 x 10^-16
5) Green light (wavelength 532 nm) is in the middle of the visible spectrum (400-700nm). As the human eye is most sensitive to light in this region, the green laser will seem much brighter than a red laser of comparable power, so there may be some justification in the manufacturer’s claim.

Question 3

What is the photoelectric effect?

Answer 3

Shining monochromatic light onto a metal will cause

electrons to be produced from the surface of the metal