Sir Issac Newton said that light travels only in a straight line. But in a burning candle, as flame is in upward direction the light from it does not travels in a straight line; it spreads. What is the reason for this?
I will turn my comment into an answer, and it will be within the classical electrodynamics.
Light point sources radiate outwardly in optical rays, straight lines with a center the point source, that is where the statement applies .
A flame is a complex light source, it is a mixture of small point radial sources in the gas/plasma of the flame. So it is the flame which spreads and is an extended light source, because of the heat, the gas/plasma of the combustion has less density and goes up. If you make a small hole in a piece of paper you will see radially spreading rays from the flame.
When you have a light source like a candle, light travels in many straight lines in all directions. Each light particle* travels in a straight line in a different direction. But there are so many of them that you can not perceive individual particles, so it seems like the light spreads uniformly. A light source where all light is emitted in the same direction is a laser.
But there is one thing Isaac Newton wasn't aware of: Gravity can bend light (or rather the space through which the light travels) which causes light to travel in curves. For more information, search for gravitational lensing.
Further, there are things like reflection, refraction and diffraction which cause light to change direction (but not really travel in curves. The straight line just makes a sudden change in direction)
*No, I am not going to go into wave-particle duality here
Does it ?
Well in Geometric Optics light is considered to be a straight ray .. But the real picture is a lot more complex ,light is actually a wavicle (YES THAT TERM EXISTS!!).
Unobstructed light does travel in a straight ,the reason you ask .. well It's unobstructed ,Try obstructing light with something like a hair and you will see it bend (It's called Diffraction) .