Question

A laser is mounted as shown in the figure (distances d and h aregiven) above the axis of a converging lens with positive focallength f. The laser beam travels parallel to the axis of the lens.A large screen is placed at a distance d_s to the right of thelens. The laser beam passes through the lens and makes a dot on thescreen at a distance h_s, measured upward from the axis of thelens. Assume that a positive value means that the dot is above theaxis, while a negative value means that the dot is below theaxis.

Find the distance h_s.
Express your answer in terms of some or all of the variables d, h,f, and d_s. Note that not all of the variables may be required forthe answer.

Now consider a specific case. Let the laser be 50 centimeters tothe left of the lens and at height 15 centimeters above the axis ofthe lens. The lens has focal length 30 centimeters, and the screenis 1 meter to the right of the lens.
Part B
What is the position h_s of the dot on the screen?
Express your answer in centimeters, to two significant figures.
Part c
If the laser is moved farther from the lens, what happens to thedot on the screen?

a)It moves up.
b)It moves down.
c)It does not move.

Part D
If the laser is moved up, farther above the axis of the lens, whathappens to the dot on the screen? Assume that the ray still strikesthe lens.

a)It moves up.
b)It moves down.
c)It does not move.

http://session.masteringphysics.com/myct

Answer 1

Part A

h_s=-(h(d_s-f))/f

Notice that the sign of the height depends uponthe sign of d_s-f. If the focal point is in front of thescreen, the dot will

be below the axis of the lens. If the focalpoint is on the screen, the image will be at the level of the axisof the lens, and if

the focal point is behind the screen, the dotwill be above the axis of the lens.

Part B

h_s=-35cm

Just use the equation above and plug itin.

Part C

The laser will not move at all. By just moving the laserfarther from the lens, it makes the dot on the screenfuzzier.

Part D

The laser will move down because it is an invertedimage.