Lenses And The Eye P
Visão Geral
Hold a magnifying glass over a printed word and the letters swell. Hold the same piece of glass a hand's width further away and the word flips upside down and shrinks. Nothing about the lens changed. You changed where you put it, and that single decision decided whether the image came out bigger or smaller, the right way up or inverted, and whether it could have been caught on a screen at all. One piece of curved glass, five genuinely different outcomes, and a rule that tells you which one you are going to get before you look.
This lesson gives you that rule and the drawing that proves it. You will learn how a convex lens gathers parallel light to a single point and how a concave lens throws it apart, how to construct an accurate ray diagram on graph paper with two lines and a ruler, and how to describe an image in the exact three words an examiner is waiting for. Then you will turn the whole thing inwards, because the most impressive lens you will ever study is about a centimetre across, changes its own shape roughly a hundred thousand times a day, and is sitting behind your pupil reading this sentence. You will see why some eyes cannot focus on the board, why the correcting lens is convex for one defect and concave for the other, what a surgeon's laser is actually doing, and why a camera has to solve the same problem in a completely different way.
Objetivos
- A lens forms an image by refracting light.
- In a convex (converging) lens, parallel rays of light are brought to a focus at the principal focus. Students should be aware of the nature of the image produced by a converging lens for an object placed at different distances from the lens, including the use of the converging lens as a magnifying glass.
- In a concave (diverging) lens, parallel rays of light diverge as if coming from the principal focus. Students should be aware of the nature of the image produced by a diverging lens for an object placed at different distances from the lens.
- The distance from the lens to the principal focus is called the focal length.
- The nature of an image is defined by its size relative to the object, whether it is upright or inverted relative to the object and whether it is real or virtual.
- Ray diagrams are used to show the formation of images by convex and concave lenses. Students may be asked to draw and interpret ray diagrams drawn on graph paper.
- The magnification produced by a lens may be calculated using the equation: magnification = image height / object height
- Our eyes only detect visible light, a limited range of electromagnetic waves. The eye contains the following structures: retina; variable focus lens; cornea; pupil/iris; ciliary muscle; suspensory ligaments. Students should know the function of each of these parts and understand how the action of the ciliary muscle causes changes in the shape of the lens that allow light to be focused arriving from varying distances. They should understand that light entering the eye is refracted by the cornea as well as by the lens.
- Usually the near point of the human eye is approximately 25 cm from the eye and the far point is at infinity. The eye can focus on objects between the near point and the far point. The distance between these points is called the range of vision.
- Lenses can be used to correct defects of vision: long sight, caused by the eyeball being too short, or the eye lens being unable to focus a sharp image on the retina; short sight, caused by the eyeball being too long, or the eye lens being unable to focus a sharp image on the retina. Students should understand the use of convex and concave lenses to rectify these defects and assist the eye to produce a focused image on the retina.
- Lasers are concentrated sources of light and can be used for cutting, cauterising and burning. Lasers can be used in eye surgery, to correct visual defects. Knowledge of how lasers work is not required.
- Comparisons can be made between the structure of the eye and the camera. In the eye the image is brought to focus on the retina by changing the shape of the lens, in a camera the image is brought to focus on the film or CCD sensor by varying the distance between the film and the lens. Students should be aware that the film (or CCD sensor) in a camera is the equivalent of the retina in the eye.
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- What happens to rays of light that arrive at a convex lens travelling parallel to the principal axis? A. They are brought to a focus at the principal focus B. They spread out as if coming from the principal focus C. They carry on without changing direction D. They are reflected back the way they came Answer: A
- An object is placed between a converging lens and its principal focus. What is the nature of the image? A. Diminished, inverted and real B. Magnified, inverted and real C. Magnified, upright and virtual D. Diminished, upright and virtual Answer: C
- An object 2.0 cm tall forms an image 5.0 cm tall. What is the magnification? A. 0.40 B. 2.5 C. 3.0 cm D. 10 Answer: B
- Which lens is used to correct short sight? A. A convex lens, because it converges the light before it enters the eye B. A concave lens, because it diverges the light before it enters the eye C. A convex lens, because it diverges the light before it enters the eye D. A concave lens, because it converges the light before it enters the eye Answer: B
- A person looks up from a book and focuses on a distant tree. What happens in the eye? A. The ciliary muscle contracts and the lens becomes fatter B. The ciliary muscle relaxes and the lens becomes thinner C. The lens moves further from the retina D. The retina moves further from the lens Answer: B
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