Chapter5 Rev Ans

Chapter 5 _Lens Revision summary

1) 2) 3) 4)

The exp set up to study – (a) u and v Ray diagrams Lens formula Essay Qs

(b) u and image size / magnification

1) EXPERIMENT

(i) (ii) (iii)

To study the relationship between the object distance, u and the image distance, v To study the relationship between the object distance, u and image size, h To study the relationship between the object distance, u and magnification, M

(a)

Inference

: The image size depends on the object distance

(b)

Hypothesis

: The shorter the object distance, the bigger the image height

(c)

Aim

: To study the relationship between the object distance, u and the image height, h

(d)

MV RV CV

(e)

Arrangement of apparatus

: The object distance, u : The image height, h : The focal length of the lens, f : A lens with fo=10 cm, ruler, lens holder, cross-wire(object), lamp with power supply and screen.

Draw a functional arrangement of the apparatus Suggested diagram :

Rohana/PracticeMakesPerfect/Rev5-spm2008

-pg 1 -

Chapter 5 _Lens State how the manipulated variable is controlled

The convex lens is placed at distance of, u = 15 cm from the object State how the responding variable is measured

The screen is adjusted until a sharp image is formed on it. The size of the image, H is measured. State how the procedure is repeated to obtain at least 5 sets of results

The procedure is repeated with values of u = 20 cm, 25 cm, 30 cm, 35 cm and 40 cm. State how the data is tabulated

u / cm

H / cm

15 20 25 30 35 40 State how the data is analysed A graph of H against u is drawn

Rohana/PracticeMakesPerfect/Rev5-spm2008

-pg 2 -

Chapter 5 _Lens 2) THE CHARACTERISTICS OF IMAGE FORMED – USING RAY DIAGRAM & LENS FORMULA Givenf = 10.0 cm; If

(i) (ii) (iii) (iv) (v)

u u u u u

= = = = =

30.0 cm 20.0 cm 15.0 cm 10.0 cm 5.0 cm

Rules Draw principal axis Draw the lens – symbol Mark the focal point, F Refer to u, draw the object From head – go straight to the lens and go to focal point, F From head – go straight to the centre and jgn belok-belok Intersection of two rays – the image

RAY DIAGRAM

LENS FORMULA

(a) Object is placed, u = 30.0 cm

Characteristics : real, inverted and diminished (b) Object is placed, u = 15.0 cm

Characteristics : real, inverted and magnified

(c) Object is placed, u = 8.0 cm

Rohana/PracticeMakesPerfect/Rev5-spm2008

-pg 3 -

Chapter 5 _Lens Object distance,u

Characteristics of the image formed

Application

Virtual, upright and magnified

Magnifying glass

Virtual, upright and formed at infinity

telescope

f < u < 2f

Real, inverted and magnified

ohp

u = 2f

Real, inverted and same size

photostat

u > 2f

Real, inverted and diminished

camera

u < f u

= f

Example 1

Example 2

A convex lens has focal length 10 cm. Determine its power.

A concave lens of power - 5D. Calculate its focal length.

Ans = 10 D

Ans = - 20 cm

Example 3 A convex lens of focal length 10 cm. An object with height 2 cm is placed 30 cm. Calculate (a) the image distance

Ans = 15 cm (c) height of the image

Ans = 1 cm diminished

(b) magnification

Ans = 0.5 times (d) characteristics of the image

Ans = Real, inverted and

Example 4 A concave lens of power 2.5 D. An object with height 6 cm is placed 20 cm. Calculate (a) the image distance

(b) magnification

Ans = - 13.3 cm

Ans = 0.67

(c) height of the image

Ans = 4.02 and diminished Rohana/PracticeMakesPerfect/Rev5-spm2008

(d) characteristics of the image

Ans = Virtual, upright

-pg 4 -

Chapter 5 _Lens 1. MODIFIED KELANTAN 2007- SECTION B

9. Diagram 9.1 and Diagram 9.2 show the parallel rays of light directed towards the convex lenses J and K. O is the optical centre and F is the focal point for each lens.

(a) (i)

What is meant by focal length?

The distance between the optical centre and the focal point [1 mark] ii) With reference to Diagram 9.1 and Diagram 9.2, compare the thickness of the lenses and the effects it has on the refracted rays to make a deduction regarding the relationship between the thickness of the lenses and their focal length.

 Lens K is thicker than lens J  Refracted rays converge at point F  The distance of OF of lens K is shorter than the distance of OF lens J  The distance between OF is the focal length of the lens  The thicker the lens, the shorter the focal length [5 marks] (b)

Diagram 9.3 shows the ray diagram of a simple microscope.

Rohana/PracticeMakesPerfect/Rev5-spm2008

-pg 5 -

Chapter 5 _Lens DIAGRAM 9.3 (i)

State the function of the eyepiece.

As magnifying glass [1 mark] (ii)

State the characteristics of the image formed by a microscope.

Virtual, inverted and magnified [3 marks] (c)

You are given two convex lenses S and Q of different focal length. Lens S has a longer focal length than lens Q.

(i)

Using the two lenses above explain how are you would make a simple astronomical telescope.

2

1

3

4 [4 marks]

(ii)

Suggest modifications that need to be done on the telescope to produce clearer and bigger images.

Use low power convex lens as the objective lens Use high power convex lens as the eye lens Use bigger diameter of convex lenses

Longer focal length of objective lens, higher magnification, M because M= fo/fe Longer focal length of objective lens, higher magnification, M because M= fo/fe More light will enter the telescope and a clearer image is seen [6 marks]

Rohana/PracticeMakesPerfect/Rev5-spm2008

-pg 6 -

Chapter 5 _Lens

2.

SECTION B - SPM 2006

Diagram 9.1 and 9.2 show the light rays from two identical objects passing through the convex lenses, J and K. Both the lenses produce virtual images. F is the focal point for each lens. Both the lenses produce virtual images. F is the focal point for each lens.

(a)

(i)

What is meant by virtual image?

Image that cannot be formed on a screen [1 mark] .

(ii)

With reference to Diagram 9.1 and Diagram 9.2, compare the thickness of the lenses, the focal and the size of image produced by the lenses J and K. Relate the size of the image with the focal length to make deduction regarding the relationship between the power of the lens and the focal length

 Lens J is thicker than lens K  The focal length of lens J is shorter than len K  The size of image produced by the lens J is bigger than lens K  The shorter the focal length, the bigger the size of the image  The shorter the focal length, the bigger the power of the lens [5 marks] (b)

Explain why a piece of paper burns when placed under a convex lens aimed towards hot sun rays.

2

Rohana/PracticeMakesPerfect/Rev5-spm2008

-pg 7 -

Chapter 5 _Lens

3 1 F 4

[4 marks] (c)

You are given two convex lenses, P and Q, with focal lengths 40 cm and 5 cm respectively. Both the lenses are used to build a simple astronomical telescope at normal adjustment. (i) Using the two lenses, explain how you are going to build the simple astronomical telescope. [6 marks] (ii)

Suggest modifications that need to be done on the telescope to produce clearer and bigger images. [4 marks]

Rohana/PracticeMakesPerfect/Rev5-spm2008

-pg 8 -

Chapter 5 _Lens

3.

SECTION C - SPM 2005 (a)

Figure 11.1 shows two convex lenses, R and S, used in an astronomical telescope. The focal length for R is 40 cm and for S is 10 cm.

(i)

Define the power of a lens.

Power is reciprocal of focal length (in metre) // 1 / focal length (in metre) [1 mark]

(ii)

Give two reasons why R is used as the objective lens of the telescope.

 Longer focal length  Bigger diameter [2 marks] (iii)

Using lenses R and S, draw a ray diagram of Figure 11.1 to show the formation of the final image by the telescope at normal adjustment. Use a scale of 10 cm to 1 cm.

Rohana/PracticeMakesPerfect/Rev5-spm2008

-pg 9 -

Chapter 5 _Lens (iv)

[5 marks] Calculate the magnification of the final image formed by the telescope at normal adjustment.

Magnification

= fo fe = 40/10 =4

[2 marks] (b)

Presbyopia is an inability of the eye to adjust the focusing of far and near objects. This usually happens when we get old, where the ciliary muscles is less able to change the shape of the eye lens to focus the image on the retina, as shown in Figures 11.2 and 11.3.

As an optician, you have three designs of lenses, X, Y and Z, in your shop. The cross-section of X, Y and Z are shown in Figure 11.4.

(i)

Choose the most suitable design of lens in Figure 11.4 to correct the sight of a person with presbyopia and explain the suitability of the design.

 Design Y is chosen  Y is a combination of convex lens and concave lens Rohana/PracticeMakesPerfect/Rev5-spm2008 -

-pg 10

Chapter 5 _Lens

 Convex lens at the bottom and concave lens at the top  To view far objects, the upper part of the lens is used  Because the convex lens increases the distance of the image  To view near objects, the lower part of the lens is used  The convex lens reduces the distance image (Any FIVE marks) (ii)

Explain why the other two designs are not suitable. Either low or high refractive index glass can be used to make the lenses.

(iii)

Which type of glass is not suitable to make the lens chosen in (b) (i)? Give reasons for your choice. [10 marks]

Rohana/PracticeMakesPerfect/Rev5-spm2008 -

-pg 11