A 35-year-old patent clerk needs glasses of 50-cm focal length to read patent applications that he holds 25cm from his eyes. Five years later, he notices that while wearing the same glasses, he has to hold the patent applications 40cm from his eyes to see them clearly.

What should be the focal length of new glasses so that he can read again at 25cm?

Express your answer to two significant figures and include the appropriate units.

Explanation:

In the second case , he wants that object placed at 40 cm be visible . For it he should wear a spec which forms image of object such  that rays  coming from 25 cm  appear  coming from 40 cm . In other words

object distance u = 25 cm

image distance v = 40 cm

f = ?

Both u and v are negative as both are in front of lens.

applying lens formula

1 / v - 1 / u = 1 /f

- 1 / 40 + 1 /25 = 1 /f

= 15 / (40 x 25 )  = 1 / f

f = (40 x 25 ) / 15

= 66.67 cm

= 67 cm

heat the gas and measure the pressure

adding a(n) to an electric circuit would increase the magnetic field around a wire;

b. solenoid

a solenoid is a type of electromagnet whose purpose is to generate a controlled magnetic field.

if the purpose of the solenoid is instead to impede changes in the electric current, a solenoid can be more specifically classified as an inductor rather than an electromagnet.