A museum conservator uses a converging lens to project an enlarged image of a small star symbol from an old glass slide onto a white screen. Fig. 1 shows th...
Question 1 Report
A museum conservator uses a converging lens to project an enlarged image of a small star symbol from an old glass slide onto a white screen. Fig. 1 shows the principal axis, a lens and the positions of its focal points. The object is placed between F and 2F on the left of the lens. The student needs to draw rays before deciding where to put the screen.
(a) Complete a ray diagram by drawing two suitable rays from the top of the object. [3]
(b) State the nature of the image formed on the screen. [2]
(c) Describe how the screen position changes if the object is moved closer to the lens but remains outside F. [1]
(d) Calculate the power of a lens with focal length 0.080 m. [2]
Answer Details
(a) A converging lens forms a real image when the object is outside its focal point. Draw one ray from the top of the object parallel to the principal axis; after the lens it passes through the focal point on the right. Draw a second ray through the centre of the lens; it continues undeviated. Where they meet is the top of the inverted image.
The rays meet on the right of the lens and the image arrow is inverted. [3]
(b) Because the rays really meet on a screen, the image is real. It is also inverted and enlarged. [2]
(c) Moving the object closer to the lens while it remains outside \(F\) makes the image distance increase. Move the screen further from the lens. [1]
(d) Lens power is the reciprocal of focal length in metres:
\[P=\frac{1}{f}=\frac{1}{0.080}=12.5\text{ D}\]
The power is 12.5 D. [2]
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