Light

Refraction of light

• Define and use the terms normal, angle of incidence and angle of refraction
• Describe an experiment to show refraction of light by transparent blocks of different shapes
• Describe the passage of light through a transparent material
• State the meaning of critical angle
• Describe internal reflection and total internal reflection using examples
• Extended tier only: Define refractive index as the ratio of the speeds of a wave in two different regions
• Extended tier only: Recall and use the equation n = sin i / sin r
• Extended tier only: Recall and use the equation n = 1 / sin c
• Extended tier only: Describe the use of optical fibres, particularly in telecommunications

Thin lenses

• Describe the action of thin converging and thin diverging lenses on a parallel beam of light
• Define and use the terms focal length, principal axis and principal focus
• Draw and use ray diagrams for the formation of a real image by a converging lens
• Describe the characteristics of an image using the terms enlarged/same size/diminished, upright/inverted and real/virtual
• Know that a virtual image is formed when diverging rays are extrapolated backwards and does not form a visible projection on a screen
• Extended tier only: Draw and use ray diagrams for the formation of a virtual image by a converging lens
• Extended tier only: Describe the use of a single lens as a magnifying glass
• Extended tier only: Describe the use of converging and diverging lenses to correct long-sightedness and short-sightedness

1. Reflection of Light

Key Terms for Reflection

When drawing reflection diagrams, we use these important terms:

• Normal: An imaginary line drawn at right angles (90°) to the surface where the light hits. We use this line to measure angles.
• Incident ray: The light ray traveling towards the surface (before reflection).
• Reflected ray: The light ray traveling away from the surface (after reflection).
• Angle of incidence (i): The angle between the incident ray and the normal.
• Angle of reflection (r): The angle between the reflected ray and the normal.

Important: Angles are always measured from the normal line, not from the surface itself.

Reflection in a Plane Mirror

A plane mirror is a flat, smooth mirror like the one you might have at home.

When you look in a plane mirror, you see an image of yourself. This image has special characteristics:

1. Virtual image: The image appears to be behind the mirror, but it's not really there. Light rays don't actually meet behind the mirror — they only appear to come from there.
2. Upright: The image is the right way up (not upside down).
3. Same size: The image is exactly the same size as the object.
4. Same distance: The image is the same distance behind the mirror as the object is in front of it.
5. Laterally inverted: Left and right are swapped. If you raise your right hand, your image appears to raise its left hand.

How to Draw a Ray Diagram for a Plane Mirror

Follow these steps:

1. Draw the object in front of the mirror
2. Draw the image behind the mirror at the same distance (use dotted or dashed lines for the image because it's virtual)
3. Draw two light rays from a point on the object to the mirror
4. Where each ray hits the mirror, draw the normal line
5. Draw the reflected rays making sure angle i = angle r for each ray
6. Extend the reflected rays backwards behind the mirror using dotted lines
7. These dotted lines should meet at the image position

Remember: Light always travels from the object to the eye, so draw your arrows pointing in this direction.

Investigating Reflection (Practical Experiment)

Aim: To prove that the angle of incidence equals the angle of reflection.

Equipment needed:

• Ray box (to produce a narrow beam of light)
• Plane mirror
• Protractor (to measure angles)
• Pencil and ruler
• Sheet of paper

Method:

1. Draw a straight line about 10 cm long in the middle of the paper
2. Use a protractor to draw a line at 90° through the middle of this line (this will be where you place the mirror)
3. Place the mirror along the first line
4. Switch on the ray box and aim a beam of light at the point where the two lines cross
5. Use a pencil to mark two points: one on the incident ray and one on the reflected ray
6. Remove the mirror and ray box
7. Use a ruler to join these points to where the lines cross
8. Measure the angle between each ray and the 90° line
9. The angle for the incoming ray is the angle of incidence; the other is the angle of reflection
10. Repeat with the light beam at different angles

Expected results: If done correctly, the angle of incidence should equal the angle of reflection each time.

Safety: Don't look directly at the ray box light. Keep liquids away from electrical equipment.