Question

1. Without corrective lenses a near sighted person can see clearly only objects that are within 100 cm from his eyes. The person has normal vision when wearing contacts. What type of lens are the contact lens, and what is their focal length?

2. A person can see clearly objects that are between 1 meter and any larger distance from their eyes. What is required to restore their vision to normal?

3. In a youngs double slit experiment, the distance from the central maximum on the screen to the first minimum is 2.0 mm. The slits are separated by d=200lambda, where lambda is the wavelength of incident light. What is the distance from slits to screen?

4. In order to increaase the separation between bright fringes on the screen of a Youngs double slit apparatus, what must you do? decrease the distance from slits to screen, decrease the slits separtation, increase slit separation, or decrease wavelength?

5. Monochromatic light is incident on a soap fil (n=1.4) from air (n=1.0). What is the apperance of the flim in reflected light as the thickness of the flim approaches zero?

Answer 1

Number 1)

We need to take objects at infinity and form images at 100 cm in front of the eyes

1/f = 1/infinity + 1/-100

f = -100 cm

That is a diverging lens

Number 2)

We need to take objects at 25 cm and form images at 100 cm

1/f = 1/25 + 1/-100

f = 33.3 cm, converging lenses

Number 3)

Apply y/L = (m + .5)(wavelength)/d

.002/L = .5(wavelength)/200wavelength

L = .80 m (80 cm)

Number 4)

The formula is y/L = m(wavelength)/d

Of the choices given, the only thing you can do is decrease slit separation. You see that d is inversely proportional to the fringe separation (y), so decreasing d increases y.

Number 5)

The formula that applies here is 2nt = m(wavelength)

As t approaches zero, m must approach zero, so we have a dark spot