Light energy emitted by stars is due to

1. Breaking of nuclei 

2.Joining of nuclei       

3. Burning of nuclei

4. Reflection of solar light

The constituents of atomic nuclei are believed to be 

1. neutrons and protons

2. protons only

3. electrons and protons

4. electrons, protons and neutrons

The half-life of radium is about 1600 yr. Of 100 g of radium existing now, 25 g will remain unchanged after 

1. 4800 yr

2. 6400 yr 

3. 2400 yr

4. 3200 yr

Half-life of a radioactive substance is 12.5 h and its mass is 256 g. After what time, the amount of remaining substance is 1 g? 

1. 75 h

2. 100 h 

3. 125 h

4. 150 h

A radioactive substance disintegrates 1/64 of initial value in 60 s. The half-life of this substance is

1. 5 s 

2. 10 s

3. 30 s 

4. 20 s

The nucleus ${}_6{}^{12}C$ absorbs an energetic neutron and emits a beta particle $\left({\beta }^{-}\right)$. The resulting nucleus is 

1. ${}_7{}^{14}C$

2. ${}_7{}^{13}N$ 

3. ${}_5{}^{13}B$ 

4. ${}_6{}^{13}C$

If in a nuclear fusion  process, the masses of the fusion nuclei be $m_1<mspace\; width="1em"></mspace>and<mspace\; width="1em"></mspace>m_2$ and the mass of the resultant nucleus be $m_3$, then 

1. $m_3=m_1+m_2$

2. $m_3=\left|m_1-m_2\right|$

3. $m_3<\left(m_1+m_2\right)$ 

4. $m_3>\left(m_1+m_2\right)$

The nuclei of which one of the following pairs of nuclei are isotones? 

1. ${}_{34}S{e}^{74},<mspace\; width="1em"></mspace>{}_{31}G{a}^{71}$ 

2. ${}_{42}M{o}^{92},<mspace\; width="1em"></mspace>{}_{40}Z{r}^{92}$

3. ${}_{38}S{r}^{84},<mspace\; width="1em"></mspace>{}_{38}S{r}^{86}$ 

4. ${}_{20}C{a}^{40},<mspace\; width="1em"></mspace>{}_{16}S_{32}$

The counting rate observed from a radioactive source at t = 0 second was 1600 counts per second and at t = 8 seconds it was 100 counts per second. The counting rate observed, as counts per second, at t = 6 seconds will be:

1. 400

2. 300

3. 200

4. 150

If $N_0$ is the original mass of the substance of half life period $T_{1/2}$= 5 years, then the amount of substance left after 15 years is

1. $N_0$/8 

2. $N_0$/16

3. $N_0$/2

4. $N_0$/4