Light (Cambridge (CIE) IGCSE Physics)

A student is determining the refractive index n of the material of a transparent block.

    Fig. 3.1 shows the outline ABCD of the transparent block.

(i) On Fig. 3.1: 

• draw a normal NL at the centre of side AB

  • continue the normal so that it passes through side CD of the block

  • label the point F where NL crosses AB

  • label the point G where NL crosses CD.

[1]

(ii) Draw a line EF at an angle i = 30° to the left of the normal and above side AB.

[1]

(iii) Mark the positions of two pins P₁ and P₂ on line EF placed at a suitable distance apart for this type of ray-tracing experiment.

[1]

The student observes the images of P₁ and P₂ through side CD of the block so that the images of P₁ and P₂ appear one behind the other.

    He places two pins P₃ and P₄ between his eye and the block so that P₃, P₄ and the images of P₁ and P₂ seen through the block, appear one behind the other.

    The positions of P₃ and P₄ are marked on Fig. 3.1.

(i) Draw a line joining the positions of P₃ and P₄. Continue the line until it meets the normal NL.

Label the point H where the line meets side CD. Draw the line FH.

[1]

(ii) Measure and record the length a of the line GH.

a = ........................................................ [1]

(iii) Measure and record the length b of the line FH.

b = ........................................................ [1]

(iv) Calculate the refractive index n using the following equation:

.

n = ........................................................ [1]

Extended tier only

The student repeats the procedure using the angle of incidence i = 45°. 

a = 3.2 cm   

b = 6.9 cm   

Calculate the refractive index n, using the equation .

n = ........................................................

The student expected the two values of refractive index n obtained in this experiment to be equal.

State two difficulties with this type of experiment that could explain any difference in the two values of n.

A student suggests precautions to take in this experiment to obtain reliable results. Tick one box to indicate the most sensible suggestion.

  Carry out the experiment in a darkened room.

  Use pins that are taller than the height of the block.

  View the bases of the pins.

  View the pins with one eye closed.   

A student investigates the position of the image in a plane mirror.

Fig. 3.1 shows the ray-trace sheet he uses.

The student draws the line MR.

• He draws a normal NL to this line that passes through the centre of MR.

• He labels the point at which NL crosses MR with the letter B.

• He draws a line from B at an angle of incidence i = 30° to the normal below MR and to the left of the normal. He labels the end of this line A.

• He places a pin P₁ on line AB, as shown in Fig. 3.1. He places another pin P₂ on the line AB.

• He places the reflecting face of the mirror vertically on the line MR.

• He views the images of pins P₁ and P₂ from the direction indicated by the eye in Fig. 3.1.

(i) On Fig. 3.1, mark with a cross a suitable position for pin P₂ in this experiment.

[1]

(ii) He places two pins P₃ and P₄ some distance apart so that pin P₃ and the images of P₂ and P₁ all appear exactly behind pin P₄. The positions of P₃ and P₄ are shown on Fig. 3.1.

Draw the line joining the positions of P₃ and P₄. Continue the line until it extends at least 7.0 cm beyond MR.

[2]

The student keeps pin P₁ in the same position but moves pin P₂ so that the angle of incidence i = 40°.

• The pin positions P₅ and P₆ for the reflected ray are marked on Fig. 3.1.

(i) Draw the line joining the positions of P₅ and P₆. Continue the line until it extends at least 7.0 cm beyond MR.

Label with the letter Y the point where the two lines cross beyond MR.

[1]

(ii) Draw a line from P₁ to MR that meets MR at a right angle. Measure and record the length a of this line.

a = ..................................................... [1]

(iii) Draw a line from the point labelled Y to MR that meets MR at a right angle. Measure and record the length b of this line.

b = ..................................................... [1]

The student removes all the pins. He places pin P₇ on the normal at a distance 6.0 cm from the front of the mirror.

• He views the image of P₇ in the mirror.

• He places pin P₈ on the normal behind the mirror.

• He adjusts the position of P₈ so that the image of the bottom of the pin P₇ and the top of pin P₈ seen over the mirror appear as one pin when viewed from all angles in front of the mirror.

(i) On Fig. 3.1, measure the distance x along the normal between P₈ and the mirror.

x = ..................................................... [1]

(ii) Complete the diagram in Fig. 3.2 to show the appearance of the image of pin P₇ and pin P₈ as described in (c).

[1]

The student expects the readings to show that the image formed in a plane mirror is the same distance behind the mirror as the object is in front of the mirror. Readings of a = b and x = 6.0 cm will show this.

State whether your readings show that the image formed in a plane mirror is the same distance behind the mirror as the object is in front of the mirror. Justify your statement by reference to the readings.             

statement:

justification:  

A student carefully carries out an investigation into an image showing the position of different pins in a plane mirror.

Suggest a practical reason why the results may not be accurate.

A student is investigating the reflection of light by a plane mirror.

  Fig. 1.1 shows his ray-trace sheet at full size.

The student carries out an initial experiment. He draws lines AB and CD as shown in Fig. 1.1. He then draws a line EF through a point N as shown in Fig. 1.1 and at an angle θ to line AB.

 The angle θ is

 measured to be 23° ±1°

Draw a normal to line AB at point N and extend the normal to line CD. Label the point at which the normal crosses line CD with the letter L.

The student places a plane mirror on line EF and a screen with a 2 mm slit on line CD. He arranges the screen so that a ray of light shines along line LN. The ray reflected from the mirror passes through point P.

 State and explain whether point P, shown on Fig. 1.1, is at a suitable distance from point N for this investigation.

(i) Draw a line joining point N and point P. Extend this line until it meets line CD.

Label the point at which this line meets line CD with the letter G.

[1]

(ii) Measure the length a of line LG.

a = ................................................... cm [1]

The student repeats the procedure for values of θ = 25°, 20°, 15°, 10° and 5°. His values for a are shown in Table 1.1.

Table 1.1

Use the values from Table 1.1 to plot a graph of a / cm (y-axis) against θ / ° (x-axis).

Suggest a possible source of inaccuracy in this experiment, even if it is carried out carefully.

A student wishes to check if his values for a are reliable.

  Suggest how he could improve the experiment, using the same apparatus, to check the reliability of his results.

A student is determining the focal length f of a lens.

Fig. 3.1 shows the apparatus.

The student places the screen a distance D = 70.0 cm from the illuminated object.

He places the lens close to the screen and moves the lens slowly away from the screen until a clearly focused image is formed on the screen.

He measures the distance u between the centre of the lens and the illuminated object.

He measures the distance v between the centre of the lens and the screen.

He repeats the procedure using values for D of 75.0 cm, 80.0 cm, 85.0 cm and 90.0 cm.

The readings are shown in Table 3.1.

Calculate, and record in Table 3.1, uv for each value of D.

Table 3.1

Plot a graph of uv/cm² (y-axis) against D/cm (x-axis). You do not need to start your axes at the origin (0,0).

Determine the gradient G of the line. Show clearly on the graph how you obtained the necessary information.

G = ........................................................

The focal length f of the lens is numerically equal to the gradient G of the graph. Write down a value for the focal length f of the lens. Give your answer to a suitable number of significant figures for this experiment.

f = .........................................................

Suggest two difficulties in this experiment when trying to obtain accurate readings.

A student is determining the focal length of a lens.

Fig. 3.1 shows the apparatus used.

The student adjusts the position of the screen until a clearly focused image is formed on the screen.

The distance v on Fig. 3.1 is measured to be 5.80 cm.

(i) Fig. 3.1 is drawn 1/5^th actual size.

Calculate V, the actual distance from the lens to the screen

V = ......................................................... [1]

(ii) With a clearly focused image formed on the screen, the actual distance from the centre of the lens to the illuminated object, U is 20.0 cm.

Calculate the focal length f₁ of the lens using the equation 

 f₁ = ......................................................... [2]

The student repeats the procedure in (a), using a different distance U. She obtains another value for the focal length f₂.

 f₂ = 12.2 cm

 Calculate the average value f_A of the focal length of the lens, using f₂ and your value for f₁ in (a)(ii). Give your answer to a suitable number of significant figures for this experiment.

  f_A = ......................................................... 

The student states that taking more measurements improves the reliability of the value obtained for f_A.

 Suggest additional values for U that you would use.

State two precautions that you would take in this experiment to obtain accurate readings.