There are multiple reasons why do Hindus light lamps in front of gods and on other occasions. In most Hindu homes, a lamp is lit at least once or twice daily before the altar of gods, especially in the evening. Some Hindus light a lamp all day and night and never let it go off. It is called Akhanda Deepa (an incessant lamp).
To know the reason why do Hindus light lamps, we have to know first on which occasions they do so.
Hindus light a lamp on the following occasions:
1. Before or after Puja.
2. In opening ceremonies.
3. In the morning and/or evening in front of an altar.
4. During Diwali festival.
5. During Navratri festival.
6. After the death of a person.
7. While welcoming special guests.
Why do Hindus Light Lamps?
Actually, there is no single answer to this question. There are multiple reasons for doing so. First, we will see all the reasons.
1. We should not keep gods in darkness.
Hindus consider the idols of gods as real gods and wash them daily and perform Puja. They offer Naivedyam to them. They consider keeping them in darkness as a bad thing. Therefore, they light lamps before the altars so that the altar is properly lighted, especially in the evenings.
2. Worship of God with five elements (Pancha Mahabhuta).
The worship of five elements, viz. fire, water, air, earth, and space is a part of Hindu worship. In old days, the lamps used were earthen. This includes the earth element. Idols are washed with water. This includes the water element. Space and air are present everywhere. Therefore, to include the fire element in Puja, they light a lamp before the deities.
Nowadays, people don’t use clay lamps in general. They use a metal lamp. Metals are found inside the earth. Therefore, this is also accepted.
3. Fire purifies everything.
Hindus believe that fire purifies everything. When we light a lamp, it purifies the environment in the house and makes the place pleasant. It is recommended to use cow ghee and cotton to light a lamp. The burning of cow ghee purifies the environment and makes the atmosphere in the room pleasant.
4. God is represented as a pillar of fire in Hindu scriptures.
In the Lingodbhava story, Lord Shiva comes out of a pillar of fire before Lord Vishnu and Lord Brahma. That pillar of fire represents the Supreme Brahman or Parmatma.
5. Ghosts are scared of light.
It is a belief that the houses where there is no light, ghosts make their permanent residences. Lighting a lamp drives away ghosts.
6. Light symbolizes knowledge and darkness symbolizes ignorance.
Lighting a lamp symbolizes that we are wiping out the ignorance and welcoming knowledge. Lighting a lamp is an auspicious thing while dousing a lamp is inauspicious.
7. The oil or ghee in the lamp symbolizes strong desires (especially negative ones) and the wick, ego.
When lit by spiritual knowledge, negative tendencies slowly exhaust and the ego also finally perishes.
8. The flame of a lamp always burns upwards.
Similarly, we should acquire knowledge to take us towards higher ideals.
9. It symbolizes the worship of the Supreme Soul with a soul.
In Hinduism, the pillar of light symbolizes the Supreme. Hence, it symbolizes the worship of Paramatma by our Atma (symbolized by the flame of a lamp). The creator is worshiped by his creation.
10. It serves as an extra light source in the night.
In ancient times when there was no electricity, it served as an extra light source. Nowadays, also it is a good source of light for a small room. If there is a power cut-off at the night, this source comes in handy.
11. To welcome Goddess Lakshmi.
Hindus believe that at the evening time, Lakshmi enters the house. Therefore, they welcome her by lighting lamps. Also, they do not sweep the house in the evening/night.
12. The light rays emitted by a lamp have a positive effect on the mood of a person.
It is not clear what is the scientific reason behind this, but it refreshes the mind of the person. Perhaps it emits some kind of electromagnetic radiation that opens up the mind and activates the sensory areas of the brain.
13. In the Vedic times, sages used to perform yajna to please gods, which fire was an essential part.
Therefore, it might be the continuation of the same practice to light a lamp before the altar to please/invoke gods.
14. Goddess Rajarajeshwari (a combined form of Durga, Lakshmi, and Saraswati) resides in the lamp.
Rules for Lighting a Lamp in front of God:
1. The person should wash his/her face, hands, and feet.
2. The material used for lamp should be of clay or metal.
3. Cow ghee should be used in the lamp. If it is not available or is unaffordable, then the oil is also good, but never use animal fats.
4. The wick should be made of cotton or cotton thread.
5. Menstruating women/girls should avoid lighting the lamp for god.
Conclusion:
We have seen that there are multiple reasons why do Hindus light a lamp. In Hinduism, there could be multiple meanings of one thing. Therefore, each individual might have his/her own reason.
Many Hindu practices have bases in the Vedas. In the Vedic times, sages used to do yajna to please/invoke gods. Fire (Agni) is a god in Hinduism who carries the offerings in the yajna to gods. Therefore, this must be the continuation of the same practice in the form of lighting a lamp before gods (fire worship).
Hindus light lamps during opening ceremonies and while welcoming guests because they consider lighting a lamp as an auspicious and positive thing. If the lamp goes off on its own, it is considered an omen.
I think the lighting of an earthen lamp after the death of a person symbolizes that the soul is still there even if the body is dead. The earthen lamp symbolizes the physical body and the flame soul.