Sun's rays fall vertically on tropic of cancer(which is 23.5 degree North latitude ) on 21 june while on 21 December rays fall vertically on tropic of Capricorn(23.5 degree south latitude).

Sun's rays never fall vertically at any point beyond the tropics.

On 21 june, Sun is over Tropic of cancer. after which it starts heading to south. On September 22, sun is vertically over the equator and keeps moving to south. And suns rays fall vertically on tropic of Capricorn on 21 December.

After which sun again starts heading towards North and reaches the equator on 21 march and again to tropic of cancer on 21 june.
This cycle keeps on repeating and sun never goes beyond the tropics.

The Sun's rays fall vertically on the Tropic of Cancer on 21st June.

Explanation:
The Tropic of Cancer is an imaginary line located at approximately 23.5 degrees north of the Equator. It marks the farthest point north where the Sun's rays can fall directly overhead at some point during the year. On 21st June, the Sun reaches its highest position in the sky and appears directly overhead at noon along the Tropic of Cancer. This phenomenon is known as the June solstice or the summer solstice in the Northern Hemisphere.

Reasons why the Sun's rays fall vertically on the Tropic of Cancer on 21st June:

1. Earth's axial tilt: The Earth's axis is tilted at an angle of approximately 23.5 degrees relative to its orbit around the Sun. This tilt is the primary reason for the changing seasons on Earth. During the northern hemisphere's summer solstice, the North Pole is tilted towards the Sun, causing the Sun's rays to fall directly on the Tropic of Cancer.

2. Sun's apparent motion: Throughout the year, the Sun appears to move along a path called the ecliptic due to the Earth's orbit around it. On 21st June, the Sun appears to reach its highest point in the sky, directly over the Tropic of Cancer. This is because the Sun's apparent path crosses the celestial equator and reaches its northernmost point, resulting in vertical rays at noon.

3. Duration of daylight: On the summer solstice, the Northern Hemisphere experiences the longest day of the year. The Sun remains above the horizon for the maximum amount of time, resulting in more hours of daylight. This is due to the Sun's rays falling more vertically on the Tropic of Cancer and spreading over a larger area, leading to an extended period of daylight.

4. Seasonal changes: The vertical rays of the Sun on the Tropic of Cancer during the summer solstice contribute to the warmest time of the year in the Northern Hemisphere. The direct overhead rays provide more concentrated solar energy, resulting in higher temperatures and the onset of summer.

In summary, the Sun's rays fall vertically on the Tropic of Cancer on 21st June due to the combination of Earth's axial tilt, the Sun's apparent motion, and the resulting seasonal changes. This phenomenon marks the beginning of summer in the Northern Hemisphere.