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1.3.8Development of Practical Skills in Physics

Using a Laser or Light Source

5 min read 8 parts Lesson 25 of 29 1 quiz

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Lesson summary

1.3.8 Using a Laser or Light Source — OCR A Level Physics A notes: Use lasers and other light sources safely and effectively in diffraction and interference experiments, and state the precautions required.Experiments on light use a variety of sources.

  • filament lamp
  • light-emitting diode
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Monochromatic

Section 1 of 8Light sources in the laboratory

Experiments on light use a variety of sources.

Filament lamp
A filament lamp emits a broad spread of wavelengths (white light) in all directions and is incoherent.
Light-emitting diode
A light-emitting diode gives a narrow band of colours.
Laser
A laser gives a narrow, intense, almost parallel beam of one wavelength, and the light is coherent: the waves have a constant phase difference.
Coherence is essential for obtaining stable interference patterns from two slits or a diffraction grating.
A typical school laser is a diode laser of power less than 1 mW (Class 2) or less than 5 mW (Class 3R) at a wavelength near 650 nm (red) or 532 nm (green).

Section 2 of 8Laser safety

A laser beam can permanently damage the retina because the eye focuses the beam to a tiny spot. The key precautions are:

1
never look directly into the beam or at its reflection from a shiny surface;
2
use only lasers of Class 2 or below, or Class 3R with strict supervision, in a school laboratory;
3
remove watches, rings and other reflective objects from the beam path;
4
keep the beam below head height, and fix the laser and target so the beam cannot wander;
5
stop the beam with a matt screen at the end of its path;
6
post a warning sign and switch the laser off when not being used.

When discussing safety in an exam, name the hazard (permanent eye damage) and give one specific control, not just ‘be careful’.

Section 3 of 8Diffraction grating experiment

A diffraction grating has many narrow slits at equal spacing d. When monochromatic light of wavelength λ falls normally on it, bright maxima (orders) occur at angles θ given by

d sin θ = nλ

What the symbols mean: d = grating spacing (m) · θ = angle of the nth-order maximum · n = order number (0, 1, 2 …) · λ = wavelength (m)

where n = 0, 1, 2, … is the order.

1
The central maximum n = 0 lies straight ahead.
2
In a laser experiment the grating is held perpendicular to the beam and a screen is placed a distance D behind it.
3
The distance x of the nth bright spot from the centre is measured, and tan θ = x / D.
4
For small angles sin θ ≈ tan θ, but for a grating the angles can be large, so use θ = tan−1(x / D) and then sin θ.

The line spacing is d = 1 / N, where N is the number of lines per metre. A grating marked 300 lines per mm has d = 1 / (3.00 × 105) = 3.33 × 10−6 m.

Section 4 of 8Double-slit and single-slit experiments

With a double slit of slit separation a and screen distance D, the bright fringes are spaced by w = λD / a.

1
This is valid when D is much larger than a and the angles are small.
2
Measuring the distance across several fringes and dividing by the number of fringe spacings improves precision.
3
Then λ = aw / D.
4
A laser with a double slit gives bright, well-defined fringes without the need for a single slit to make the light coherent,
  • which is required with a filament lamp.

A single slit gives a central bright maximum with width twice that of the secondary maxima, whose width increases when the slit is narrowed. The first minimum occurs at the angle where a sin θ = λ for a slit width a.

Section 5 of 8Improving the experiment

1
Use as large a screen distance D as practical, so the pattern is spread out and the percentage uncertainty in measured distances is small.
2
Measure across several fringes or from the nth order on one side to the nth order on the other, then halve.
3
Darken the room to see the pattern clearly.
4
Repeat with different orders and with the grating rotated to check consistency, taking a mean value of λ.
5
Measure D with a metre rule or tape and x with a ruler, remembering that the uncertainties of both add as percentages.

This topic links to waves and interference in later units and to the measurement ideas in 1.3.1 and 1.3.2.

Section 6 of 8Worked examples

  1. Light from a laser passes through a grating with 300 lines per mm. The second-order maximum is at 23.0° from the central beam. Find the wavelength.

  2. A double slit has slit separation 0.50 mm. The fringes on a screen 2.40 m away are 3.1 mm apart. Find the wavelength.

  3. What is the maximum order seen with a grating of line spacing 2.0 μm and light of wavelength 600 nm?

Section 7 of 8Key terms

TermMeaning
CoherentWaves with a constant phase difference between them.
MonochromaticLight of a single wavelength.
Diffraction gratingSurface with many equally spaced slits, giving sharp interference maxima.
Order (n)Integer labelling each bright maximum from the central one.
Fringe spacingDistance between adjacent bright fringes.
Class 2 laserLow-power laser safe for brief accidental exposure but still never to be viewed directly.

Section 8 of 8Exam technique

1
Looking at the laser beam or its reflections; the hazard is permanent retinal damage.
2
Using sin θ ≈ tan θ for large angles with a diffraction grating.
3
Forgetting to convert lines per mm to a slit spacing in metres.
4
Measuring one fringe spacing instead of a distance across several fringes, giving a large percentage uncertainty.
5
Thinking a filament lamp gives a clear double-slit pattern without a single slit to supply coherence.
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