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Chapter 35: Q.24 (page 1017)

A scientist needs to focus a helium-neon laser beam (λ=633nm)to a 10-μm-diameterspot 8.0cmbehind a lens

(a) what focal-length lens should she use?

(b) what minimum diameter must the lens have?

Short Answer

Expert verified

(a) Focal-length lens she uses is S'=f=8cm.

(b) The minimum diameter is D=0.012m→1.2cm.

Step by step solution

01

Part (a) Step 1: Given information 

We need to find the focal length.

02

Part (a) Step 2: Simplification 

Parallel rays always focus at focal length,

Therefore, S'=f=8cm.

Here fis focal length.

03

part (b) Step 1: Given Information

We need to find the minimum diameter of the lens.

04

part (b) step 2: Calculation

By using,

ω=2.44λfD (By multiplying both sides with D.)

D×ω=D×2.44λfD

D×ω=2.44λf (Divide both the sides by ω.)

D×ωω=2.44λfω

D=2.44λfω (substitute values in equation.)

D=2.44×6.33×109×0.0810×10-6

D=0.012m→1.2cm.

Here, Dis the diameter, fis focal length and λis the wavelength.

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Most popular questions from this chapter

A common optical instrument in a laser laboratory is a beam expander. one type of beam expander is shown in FIGURE P35.28.

a. For what lens spacing d does a parallel laser beam exit from

the right?

b. What is the width of the exiting laser beam?

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