Light: reflection and refraction required practical

Aim: To investigate how light reflects from different surfaces and how it refracts when it passes through different transparent materials, using a ray box.

Variables

• Independent: Angle of incidence (and the surface or material used)
• Dependent: Angle of reflection or angle of refraction
• Control: Width of the light ray; Using the same ray box and brightness; Measuring angles from the normal every time; Same block or mirror for one set of readings

Equipment

• Ray box with a slit to give a narrow beam
• Power supply for the ray box
• Plane mirror and mirror holder
• Rectangular blocks of glass, Perspex (acrylic) and a clear container of water
• Protractor
• Ruler and sharp pencil
• Sheets of white A3 paper

Method

1. Work in a darkened room so the ray is easy to see.
2. Reflection: draw a straight line on white paper, stand the mirror on it and draw a normal line at 90° to the mirror.
3. Aim a narrow ray at the point where the normal meets the mirror, marking the incoming and reflected rays with crosses.
4. Remove the mirror, join the crosses with a ruler and measure the angle of incidence and angle of reflection from the normal.
5. Refraction: place a glass block on fresh paper and draw round it.
6. Shine the ray into one long side at an angle, and mark the ray going in and the ray coming out with crosses.
7. Remove the block, join the points to show the path through the block, draw the normal where the ray enters and measure the angles of incidence and refraction.
8. Repeat for several angles of incidence, then repeat with the other materials and compare the results.

Safety

• The ray box gets hot: do not touch the bulb housing and let it cool before packing away.
• Take care moving around in a dim room; keep bags and stools out of walkways.
• Handle glass blocks carefully, as chipped edges can cut.

Results and calculations

Record the angle of incidence and the angle of reflection or refraction for each material in a table. For reflection, the angle of incidence equals the angle of reflection; for refraction, light bends towards the normal on entering a denser material, and the material that bends it most has the smallest angle of refraction for the same angle of incidence.

Common mistakes and improvements

• A wide or blurry ray makes angles hard to measure: use a narrow slit, a dark room and mark the centre of the ray.
• Measuring angles from the surface instead of the normal: always draw the normal first and measure from it.
• The block moves during the experiment so the ray path is wrong: draw round the block and check it is still in position before marking points.
• Marking the ray with points too close together makes the line inaccurate: space the crosses far apart along each ray.

Exam tips

• Be able to draw ray diagrams with a ruler, arrows on rays, a dashed normal and labelled angles.
• Explain refraction in terms of a change in speed: light slows down in glass, so it bends towards the normal.
• Know that a ray hitting the boundary along the normal (at 0°) does not change direction.
• Mention specular reflection from smooth surfaces and diffuse (scattered) reflection from rough surfaces.
• When suggesting improvements, say you would repeat each angle and use a narrower beam.