A wildlife photographer has difficulty focusing on a hawk flying far away after spending an hour editing close-up pictures on a screen. Fig. 1 shows the eye...
Question 1 Report
A wildlife photographer has difficulty focusing on a hawk flying far away after spending an hour editing close-up pictures on a screen. Fig. 1 shows the eye when viewing the close screen. Ciliary muscles are shown around the lens. The photographer's eye automatically changes the shape of the lens so that light from the object is focused on the retina. This automatic adjustment is called accommodation. The photographer then looks from the screen to the hawk without moving the head.
(a) Describe the shape of the lens when the screen is close. [1]
(b) Explain how the ciliary muscles and lens change when the photographer looks at the distant hawk. [3]
(c) Give the name of the process described. [1]
A technician calibrates a projector used to display a large safety diagram in a factory training room. Fig. 1 shows the object slide, a convex lens and a wall screen. The slide is placed beyond twice the focal length of the lens. The diagram on the screen must be sharp enough for workers to read warning signs from the back of the room. The technician adjusts the lens position along a rail before the session begins. Light from the bright slide passes through the lens and forms an image on the wall.
(a) Describe the orientation of the projected image. [1]
(b) Explain why the wall must be placed at the position where rays meet. [2]
(c) Give two changes needed if the image on the wall is not sharp. [2]
Answer Details
Eye accommodation
- (a) For a close screen, the lens is thick or more curved. [1] A more curved lens refracts the strongly diverging rays from a nearby object sufficiently to focus them on the retina.
- (b) To view the distant hawk, the ciliary muscles relax. [1] The lens then becomes thinner or less curved. [1] Its focal length increases, so light from the distant object remains focused on the retina. [1]
- (c) This automatic change in lens shape to focus at different distances is called accommodation. [1]
Projector
- (a) The projected image is inverted. [1]
- (b) A convex lens produces a real image where the rays actually meet. [1] The wall must be at that position because a screen only forms a sharp image where the rays meet. [1]
- (c) Any two suitable adjustments are: move the lens, move the wall or screen, or adjust the object-slide position. [2] Each change alters the relative object, lens and image distances until the image plane coincides with the wall.
Examination reminder: for a distant object, the eye needs a thinner lens and a longer focal length; a screen can receive only a real image.
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