Question

Ravi is given lenses with powers +5D,-5D,+10D and -20D. considering a pair of lenses at a time,which two lense will he select to have a combination of total focal length when two lenses are kept in contact in each case.
(i)-10cm (ii)20cm (iii)-20cm

Answer 1

Hey There

Focal length = 100/power

(i) -10 =100/power

power =-10

so we can have a +10D and a -20 D lens

(ii) 20 = 100/Power

power = 5

so a +10D lens and a -5D lens

(iii) -20 =100/Power

power =100/-20

= -5

There is no pair for this focal length

But we can take 3 at a time

i.e -20D ,+10D and +5D



thanks

Answer 2

Answer:

Ravi will select lenses with powers +10D and -20D to obtain a combination of -20cm.

Explanation:

Ravi is given lenses with powers +5D, -5D, +10D, and -20D. He needs to select two lenses to obtain a particular combination of total focal length when the two lenses are kept in contact. Let's consider the given options one by one:

(i) -10cm: To obtain a combination of -10cm, Ravi needs to select lenses with powers -5D and -20D. The formula to calculate the power of two lenses kept in contact is P = P1 + P2 - (d/P1P2), where P1 and P2 are the powers of the lenses and d is the distance between them. On substituting the values, we get P = -5D -20D - (0/-5D-20D) = -25D. The focal length f = 1/P, which gives f = -40cm. Therefore, this option is not possible.

(ii) 20cm: To obtain a combination of 20cm, Ravi needs to select lenses with powers +5D and -5D. Substituting the values in the formula, we get P = +5D -5D - (0/+5D*-5D) = 0. The focal length f = 1/P is infinity. Therefore, this option is also not possible.

(iii) -20cm: To obtain a combination of -20cm, Ravi needs to select lenses with powers +10D and -20D. Substituting the values in the formula, we get P = +10D -20D - (0/+10D*-20D) = -10D. The focal length f = 1/P is -10cm. Therefore, this option is possible.

Hence, Ravi will select lenses with powers +10D and -20D to obtain a combination of -20cm.