Home Page Textbooks Physics: Principles and Problems Section 18.3: Applications of Lenses

Physics: Principles and Problems

9th Edition

Elliott, Haase, Harper, Herzog, Margaret Zorn, Nelson, Schuler, Zitzewitz

ISBN: 9780078458132

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Section 18.3: Applications of Lenses

Exercise 31

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The cornea’s role is to refract the light and to focus on objects we are looking at. It behaves like a lens and has its index of refraction. The difference between the index of refraction of air and cornea is bigger than the difference between indexes of any other part of the eye.

Result

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The difference between the index of refraction of air and cornea is very big.

Exercise 32

Solution 1

Solution 2

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Normal eye refracts light rays so they meet at the end of the eye bowl.
Light rays refracted in nearsighted eye meed before the end of an eye bowl so nearsighted person needs a lens that prolongs the angle of incidence. This is achieved with a concave lens.

The far-sighted eye, on the other hand, needs a lens that will shorten the distance from where the refracted rays meet. So a far-sighted person needs convex lenses to see better. Exercise scan

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In this problem, we are asked to state which kind of lens a nearsighted person needs to correct his sight, and which kind of lens a shortsighted person needs to do the same.

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For a person to see properly, light rays refracted by the eye lens should focus on the retina.

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**Nearsighted person**

In a person with **nearsightedness**, these rays focus in front of the retina, so a **concave** lens is needed to diverge the light rays, so that they focus farther along, on the retina.

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**Farsighted person**

In a person with **farsightedness**, these rays focus behind the retina, so a **convex** lens is needed to converge the light rays, so that they focus earlier, on the retina.

Result

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Nearsighted – concave

Farsighted – convex

Exercise 33

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When the object that we are looking at is focused, it is a real image. Real images are always farther than the focal point, when we are looking through the lens.

To make the tree a real image and to move it more farther to lens, we need to move lens closer to the film.

Result

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We need to move lens closer to the film.

Exercise 34

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When an object is placed at a distance greater than two focuses ${d_o}>2cdot f$, a real image is smaller and inverse. Reason for that is collision of light rays who meet on the other side of a principal axis.

Result

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Because a distance of an object is greater than two focuses.

Exercise 35

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Binoculars contain two prisms in each ocular. Their role is to extend the light’s path length so two oculars can be close together. If there is only one prism and lens or mirror, the image will be inverted so its role is to invert light rays two times so the viewer can see an upright image. Also, prisms improve the three-dimensionality of the image.

Result

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To extend the light’s path length, to invert an image, to improve the three-dimensionality of the image.

Exercise 36

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The magnification is inversely proportional to the image brightness. When looking through the ocular of a microscope there is a fixed amount of light and if we increase magnification, the amount of light will decrease.

We can increase the brightness of the image by adding more light respectively using a brighter lamp.

Result

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The magnification is inversely proportional to the image brightness.

By using a brighter lamp.

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Section 18.3: Applications of Lenses

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