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Chapter 25: Q43PE (page 931)

How far from a piece of paper must you hold your father’s 2.25 Dreading glasses to try to burn a hole in the paper with sunlight?

Short Answer

Expert verified

The focal point to burn a hole in the paper with sunlight isf=444mm.

Step by step solution

01

Concept Introduction

The focal length is the distance between a convex lens or a concave mirror and the focal point of a lens or mirror. It is the point at which two parallel light beams meet or converge. Depending on the lens and mirror (concave or convex), the focal length varies with the sign (positive or negative).

02

Information Provided

Power of the reading glass: 2.25 D

03

Focal length of the glasses

To burn the paper, the paper has to be at the focal point of the lens. The power in dioptres of lenses is inversely proportional to the focal length;

P=1f.

Rearranging the expression and substituting the values –

f=1P=12.25D=0.444m=0.444×1000mm=444mm

Therefore, the value for focal length is obtained as f=444 mm.

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Most popular questions from this chapter

A narrow beam of white light enters a prism made of crown glass at a 45.0° incident angle, as shown in Figure 25.57. At what angles, θRand θV, do the red (660nm) and violet (410nm) components of the light emerge from the prism?

Some telephoto cameras use a mirror rather than a lens. What radius of curvature mirror is needed to replace a800mmfocal length telephoto lens?

Is it possible that total internal reflection plays a role in rainbows? Explain in terms of indices of refraction and angles, perhaps referring to Figure 25.48. Some of us have seen the formation of a double rainbow. Is it physically possible to observe a triple rainbow?

At what minimum angle will you get total internal reflection of light traveling in water and reflected from ice?

(a) At the end of Example 25.4, it was stated that the critical angle for light going from diamond to air is 24.4º. Verify this.

(b) What is the critical angle for light going from zircon to air?
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