# Ten Questions: Electricity and Ray Optics.

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1. If 200 volts are put across a 300-loop transformer's primary coil:

a.What will be the voltage output if the secondary coil had 750 loops?

b.What is the power in the secondary coil if the resistance in this coil is 50 ohms?

c.What is the current drawn in the primary coil?

d.What is the power in the primary coil?

2. A model electric train requires 6 volts to operate. If the primary coil of its transformer has 240 windings, how many windings should the secondary have if the primary is connected to a 120-volt household circuit?

3. How does the current in the secondary of a transformer compare to the current in the primary if the voltage in the secondary is three times the voltage in the primary?

4. Define the following terms:

a. Critical angle

b. Total internal reflection

c. Penumbra

d. Umbra

5. Distinguish between reflection and refraction

6. Distinguish between a converging lens and a diverging lens.

7. When a fish looks up at an angle of 45 degrees with the horizon, does it see the sky or the reflection of the bottom beneath? Explain why.

8. Compare the characteristics and functions of the rods and the cones of the eye.

9. Distinguish between a virtual image and a real image.

10. What kind of lens can produce a real image? A virtual image?

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#### Solution Summary

Ten question related to electricity and light are answered.

Electronic and Optoelectronic Devices

1. A microscope has an objective with f = 1 cm, an ocular with f = 2.5 cm, and a distance s = 10 cm between them. Taking the best viewing distance as 25 cm,

a) Find the magnification of a microscope

b) Plot the picture of the rays to scale using the graph paper.

2. Model a parabolic graded-index fibre with n (r) = n 1 [1-(r/a)^2 delta] for r is less than or equal to a , with n 1 = 1.5, a = 0.1 mm and delta = 0.01 , by the equivalent multiple-step approximation.

Divide the radius into 10 equal parts. Consider a ray inside the fibre crossing the fibre axis at 5 degrees with respect to that axis. Using the graph paper, plot the progress of the ray through the fibre until it turns back and re-crosses the fibre axis. (Angles can be magnified on the plot.) At what value of r/a will the ray turn back? Does this depend on the value of a?

3. Find the superposition of two plane waves of equal amplitudes which have close but not equal frequencies. Plot a sketch of the resulting wave. Hint: Take the angular frequency of the waves as ω + Δω and ω - Δω, respectively. Consider the superposition at the point x = 0 and neglect the phase difference.

4. In a step-index fibre, the refraction coefficients of the core and the cladding layer are 1.500 and 1.505, respectively. Provide explanations.

(a) Find the radius of the core, at which the fibre would perform as a single-mode at the wavelength of 1.3 um, using a typical design criterion of v = 2

(b) Find the cut-off wavelength for this fibre.

5. Explain in your own words the difference between photoluminescence and electroluminescence.

6. Explain, why an LED operating at the fixed current would be more stable than that operating at the fixed voltage. Consider the effect of the device heating on the I(V) characteristics. Explain under which condition there would be positive or negative feedback from the heating.

7. A laser diode has a threshold current of 10mA and a slope of its optical power versus input current of 2mW/mA. The diode current is I = A + B sin ωt. Plot the output power as a function of time.

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