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That's a more complicated question than you think, as electromagnetic radiation (light) behaves both like a particle (called a photon) and a wave. It does travel on a straight line, unless deviated by gravity or magnetic fields and even then you could argue it's still travelling in a straight line through space in the case of gravity deviation.

It is actually unclear.... Wave/Particle duality and all that.

Lights travels in straight line "rays" and is quantized in discreet packets called photons. Light also has wave-like behavior at the quantum level.

However, the emission source, such as a light bulb or star, often emits rays in many or all directions. It is quite difficult to emit rays in only one discreet thin beam direction. Light also scatters, re-radiates, reflects, refracts, and diffracts when traveling through a dense matter medium such as our atmosphere. Which is why turning on a flashlight in a dark room illuminates all of the walls around you and behind you.

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Light Basics

Reflection, Refraction, Diffraction

Emission and Absorption

Light is both a particle and a wave. When it travels through space it is both simultenously. The particle is called the photon.

You can think of light as a wave even from light bulbs. It is always a particle and a wave simultaneously.

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