4️⃣ PAG8: Investigate light waves reflecting, refracting and being absorbed.

Question 1

Equipment

Answer 1

• Ray box
• Low voltage power supply
• Single slit
• Plane mirror
• Holder for mirror
• Protractor
• White paper
• Pencil
• Glass/Perspex block

Question 2

Health + safety

Answer 2

• Use a low voltage power supply for the ray box so not to blow the bulb
• Turn off switch between readings to ensure ray boxes don’t get too hot
• Be aware that ray boxes can get very hot so do not touch the ray box until they are cooled down

Question 3

Method 1 - reflection

Answer 3

1. Set up apparatus as shown below; do not turn on the power pack.

Set up a ray box, slit and lens so that a narrow ray of light is produced.
Place a 30 centimetre (cm) ruler near the middle of a piece of plain A3 paper. Draw a straight line parallel to its longer sides. Use a protractor to draw a second line at right angles to this line. Label this line with an ‘N’ for ‘normal’.
Place a plane mirror against the first line.

2. Turn on the power pack at the voltage suggested by your teacher. Line up the ray of light so that it is at a 90° angle to the mirrors surface. At this angle the ray should reflect back on itself.
3. Use a pencil to draw a dashed line along the ray and a straight line to show the position of the mirror. Label the dashed line as the normal.
4. Move the ray box so that the ray of light now hits the mirror at the point where the normal meets the mirror but at an angle so the light is reflected off. Draw over the path of the light with a pencil.
5. Measure the angle of incidence (the angle at which the light hits the mirror from the normal) and the angle of reflection (the angle at which the light is reflected off the mirror from the normal). Record measurements in the table below.
6. Repeat steps 4 and 5 till you have measurements for a least 5 angles. Record all results in the table below.

Question 4

Reflection - evaluation

Answer 4

The light rays should obey the law of reflection for every attempt. To what extent do the results show this?

Law of reflection = In reflection at a surface, the angle of incidence equals the angle of reflection.

Question 5

Risks/hazards 2 - reflection

Answer 5

Ray box gets hot Minor burns Do not touch bulb and allow time to cool

Semi-dark environment Increased trip hazard Ensure environment is clear of potential trip hazards before lowering lights

Question 6

Method 2 - refraction

Answer 6

1. Set up apparatus as shown below; do not turn on the power pack.

(Set up a ray box, slit and lens so that a narrow ray of light is produced.
Place a 30 centimetre (cm) ruler near the middle of a piece of plain A3 paper. Draw a straight line parallel to its longer sides. Use a protractor to draw a second line at right angles to this line. Label this line with an ‘N’ for ‘normal’.
Place the longest side of a rectangular acrylic polymer block against the first line. With the normal near the middle of the block, carefully draw around the block without moving it.)

2. Turn on the power pack at the voltage suggested by your teacher. Draw around the block in pencil. Line up the ray of light so that it is at a 90° angle to the rectangular blocks surface. At this angle the ray should pass through the rectangular block without refracting.
3. Use a pencil to draw a dashed line along the ray. Label the dashed line as the normal.
4. Move the ray box so that the ray of light now hits the block at the point where the normal meets the block but at an angle so the light is refracted through. Draw over the path of the light with a pencil.
5. Measure the angle of incidence (the angle at which the light hits the rectangular block from the normal) and the angle of refraction (the angle at which the light is refracted through the black from the normal). Record measurements in the table below.
6. Repeat steps 4 and 5 till you have measurements for a least 5 angles. Record all results in the table below.

Question 7

A student shines a light from a ray box onto a mirror. The image below show the path of the ray as it leaves the ray box and hits the mirror. Draw on the reflected ray. [2 marks]

Answer 7

Line reflected off the mirror with arrow head in correct direction 
Angle of reflection same as angle of incidence (judged by eye) 

Question 8

A periscope is used in a submarine to allow people to see above the water’s surface. A simple diagram of a periscope is shown below. Complete the diagram to show the path of the light and write down how the periscope allows the user to see above the water. [4 marks]

Answer 8

Straight line drawn from entrance to top mirror, reflected down to second mirror and into eye 
Arrow head on rays in correct direction 
Mirrors are at 45° angles  so light is reflected off the first mirror onto the second and then into the observers eye 

Question 9

3 (a) A student is investigating refraction of light. They shine a light through a rectangular glass block at an angle. The diagram below shows the ray as it hits the glass block. Complete the diagram to show the path of the ray and label the angle of incidence and angle of refraction. [2 marks]

Fill in the gaps using words below: [2 marks]
smaller larger closer to further from
The light is now passed through a more dense material. This means the angle of refraction is now __ and therefore ___ the normal.

Answer 9

line drawn so ray bends towards normal 
Both angles correctly labelled 

Smaller
Closer to

Question 10

4 A sound wave travels from water into air. Its wavelength in air is longer than in water.
Fill in the gaps to show how the frequency and speed of the wave in air compares with its value in water. [2 marks]

The frequency in air is __ as the frequency in water.

The speed in air is __ than the speed in water.

Answer 10

Same

Faster

Question 11

Note

Answer 11

‘Being absorbed’ ????