46. Which of the following statements about a neutral atom is correct? The

**A.**core is composed of electrons and protons**B.**number of electrons is equal to that of neutron**C.**number of neutrons is equal to that of protons**D.**number of protons is equal to that of electrons

**Correct Option: Answer is B**

**47. **Which of the following properties is not exhibited by sound waves

**A.**Diffraction**B.**Polarization**C.**Interference**D.**Reflection

**Correct Option: Answer is C**

**48. **When ultraviolet light is incident on certain metallic p articles are emitted. These particles are called

**A.**positrons**B.**protons**C.**photoelectrons**D.**photons

**Correct Option: Answer is D**

**49. **The graph above illustrates the variation of temperature θwith time (t) for a solid that is being heated. Which processes take place at segments P and Q respectively?

**A.**Freezing and vaporization**B.**Evaporation and solidification**C.**Melting and boiling**D.**Condensation and evaporation

**Correct Option: Answer is C**

### PHYSICS THEORY PAST QUESTIONS 2019 WITH SOLUTION

**35. **A ray of light travelling from a rectangular glass block of refractive index 1.5 into air strikes the block at an angle of incidence of 30. Calculate its angle of refraction.

**A.**48.6°**B.**19.5°**C.**20.0**D.**45.0

**Correct Option: Answer is A**

n = sinI/sinr=sin30^{o}/sinr

1.5 = sin30^{o}/sinr

1.5 sin r = sin30^{o}/1.5 = 0.5/1.5

sin r = 0.3333

= 48.6^{o}

- (a) (i) Define atomic spectra.

(ii) Differentiate between emission spectra and absorption spectra.

(b)

The diagram above illustrates an electron transition from energy level n = 3 to n = 1. Calculate the:

(i) energy of the photon

(ii) frequency of the photon

(ii) wavelength of the photon [h = 6.6 x 10

c)(i)Differentiate between soft x-rays and hard x-rays (ii) Draw the circuit symbol for a p-n junction diode (iiii) Give the reason for doping a semiconductor material^{−34}J s, c = 3.0 x 1088 ms^{−1}; 1 ev = 1.6 x 10^{−19}J]- (a) (i) Define atomic spectra.

**Solution & Explanation:**

(a) The emission of light from a substance when its excited electrons move to a lower energy level.

OR

Distinct/separate bright lines of light of definite frequency/wavelength (emitted from vapourised atoms)

(ii) Emission spectra is the pattern of light displayed when electrons jump from a higher energy level to a lower energy level, while absorption spectra is the displayed pattern of wavelengths absorbed by the gas.

(b) ΔE = Ei – Ef

= -1.51 + 13.6

= 12.09 eV

= 12.09 x 1.6 x 10^{−19} J

= 1.934 x 10−^{18} J

(ii) ΔE- hf

f = ΔE/h = 1.934×10^{−18}/6.6×10^{−34}

f = 2.93 x 10^{15}Hz

(ii) c = fλ

λ= 3×10^{8}/2.93×10^{13}

= 1.023 x 10^{−7}m

(c) (I) Soft x-rays are those with longer wave lengths/lower penetrating/less energy while hard x-rays are those with shorter wavelengths/higher penetrating power more energy.

(iii) To increase the electrical conductivity/reduce electrical resistivity of the semiconductor.

**2. **You are provided with a stopwatch, a meter rule, a split cork, retort stand and clamp, a pendulum bob, a piece of thread, and other necessary apparatus.

- Place the retort stand on a laboratory stool. Clamp the split cork.
- Suspend the pendulum bob from the split cork such that the point of support
**P**of the bob is at height**H**= 100cm above the floor**Q**. The bob should not touch the floor and**H**should be kept constant throughout the experiment.

iii. Adjust the length of the thread such that the center **A **of the bob is at a height y= **AQ**= 20cm from the floor.

- Displace the bob such that it oscillates in a horizontal plane.
- Take the time t for 20 complete oscillations.
- Determine the period T of oscillation and evaluate T

vii. Repeat the procedure for four other values of y = 30cm, 40cm, 50cm, and 60cm. In each case, determine **T **and **T**.

viii. Tabulate the results.

- Plot a graph of
**T**on the vertical axis and y on the horizontal axis, starting both axes from the origin (0,0). - Determine the slope, s, of the graph and the intercept c on the vertical axis.
- If in this experiment SR= c, calculate R.
- State two precautions taken to ensure accurate results.

(b) i. The bob of a simple pendulum is displaced a small distance from the equilibrium position and then released to perform simple harmonic motion Identify where its:

(∝) kinetic energy is maximum

(β) acceleration is maximum

- An object of weight 120N vibrates with a period of 4.0s when hung from a spring. Calculate the force per unit length of the spring. [g= 10ms
^{−2}, π=3.142]

**Correct Option: **

**3. **You are provided with a converging lens and holder, a screen, a ray box containing an illuminated object pin, and a meter rule.

- Place the lens in its holder such that it is facing a distant object seen through a well-lit laboratory window. Move the screen to and fro until a sharp image of the distant object is formed on it. Measure the distance,
**f**00, between the screen and the lens. - Clamp the meter rule securely to the table. Place the illuminated object pin at the end
**R**of the meter rule. - Place the lens at a position
**P**such that**X = RP =**20cm. - Move the screen to a position
**Q**to receive a sharp image of the object. Measure the distance**Y = PQ**. - Evaluate
**Z = (X+Y)** - Repeat the procedure for five other values of x = 25cm. 3Ocm, 35cm, 40cm and 45cm. In each case, record X,Y and evaluate Z.
- Tabulate the results.
- Plot a graph with
**Z**on the vertical axis and**X**on the horizontal axis. Draw a smooth curve through the points. - Determine from your graph the minimum value of Z=Z00 and its corresponding distance
- Evaluate W = ½ (Z04+X02Z04+X02)
- State two precautions taken to ensure accurate results.
- (b) i. Draw a ray diagram to show how a Convex lens forms an image of magnification less than one.
- Name two pairs of features in the human eye and a lens camera that performs similar functions.

H = 100cm

Table of value

SN | y(cm) | t(sec) | T | T22 |

1 | 20 | 50.50 | 2.525 | 6.376 |

** **

Scale: Let 2cm represent 1 unit on the vertical axis and 1.5cm represent 10 units on the horizontal axis.

**Precautions**

**– **Avoided parallax error in reading stopwatch/clock/ meter rule.

– Noted/corrected/avoided zero error on stopwatch/ clock/meter rule.

– Avoided draught/switch off fans.

– Avoided conical oscillation Ensured that support was rigid/firm.

– Ensured bob of the pendulum was free from table/did not touch table

– Repeated reading shown on the table.

– Small angular displacement

(b) (∝) The kinetic energy is maximum at the equilibrium position

(β) The acceleration is maximum at the point of maximum displacement.

- The period T is given by

F = 13.0cm

Table of value

S/N | Xcm | Ycm | Z(x+y)cm |

1 | 20 | 60 | 80 |

Let 1cm represent5 units on the vertical axis and 1cm represent 5 units on the horizontal axis.

Minimum value of Z=Z00 from the graph = 60cm

Corresponding distance of X00 = 30cm

- Evaluate W = ½ (Z
_{0}/4+X_{0}/2)

W = ½ (60/4+30/2)

= ½(30) = 15

**Precautions**

– Coaxial arrangement of optional instruments/ray box, len and screen on a straight line.

– Avoided parallax error in reading metre rule.

– Lens kept uptight

– Repeated reading shown on table

– Surface of lens cleaned

– Noted/corrected/avoided zero error on metre rule.

(b)i.

- Features performing the same function

Human eye | Lens camera |

Retina | Film/screen |

Eye lens | Camera lens |

Iris | Diaphragm |

Pupil | Aperture |

**4. **You are provided with a battery of e.m.f. **E**, a key **K**, a voltmeter, a standard resistor **R**00= 2Ω, a resistance box **R, **and some connecting wires.

- Measure and record the e.m.f.
**E**of the battery. - Set up a circuit as shown in the diagram above with the key open.
- Set the resistance on the resistance box to
**R**22. - Close the key, read and record the potential difference
**V**on the voltmeter. - Evaluate V−1−1
- Repeat the procedures for five other values of
**R =**5Ω 10Ω,12Ω,15Ω and 20Ω. In each case, record**V**and evaluate**V**^{−1} - Tabulate the results.
- Plot a graph with
**R**on the vertical axis and V−1 l on the horizontal axis, starting both axes from the origin (0,0). - Determine the slope,
**s**, of the graph and the intercept c on the vertical axis. - Calculate ∝and β from the equations s =
**R**0 ∝ and c= – (R00+B). - State two precautions taken to obtain accurate results.

- You are provided with a battery of e.m.f.
**E**, a key**K**, a voltmeter, a standard resistor**R**00= 2Ω, a resistance box**R,**and some connecting wires. - Measure and record the e.m.f.
**E**of the battery. - Set up a circuit as shown in the diagram above with the key open.
- Set the resistance on the resistance box to
**R**22. - Close the key, read and record the potential difference
**V**on the voltmeter. - Evaluate V−1−1
- Repeat the procedures for five other values of
**R =**5Ω 10Ω,12Ω,15Ω and 20Ω. In each case, record**V**and evaluate**V**^{−1} - Tabulate the results.
- Plot a graph with
**R**on the vertical axis and V−1 l on the horizontal axis, starting both axes from the origin (0,0). - Determine the slope,
**s**, of the graph and the intercept c on the vertical axis. - Calculate ∝and β from the equations s =
**R**0 ∝ and c= – (R00+B). - State two precautions taken to obtain accurate results.