Property of Light: Refraction & Angle of Bending

[thecosmos123456]

the basic property of light is that ,it travels in a straight line but what causes it to travel in a straight line .in refraction of light ,what causes the refracted ray to bend towards the normal ,i am aware that it is related to velocity n material of medium but how does it influence the angle of refraction

 

[Nugatory]

This is what classical wave optics is all about.

These behaviors of light can be calculated from first principles using the wave equation derived from Maxwell's laws of electricity and magnetism. The Huygens wavelet model is an easier way of getting an intuitive understanding of what's going on here (google for "Huygens refraction", and pay particular attention to the images).

 

[jfizzix]

If nothing interacts with the photon, conservation of momentum tells us it will travel (approximately) in a straight line.

When light does interact with a material, like a block of glass, the light bends because it travels more slowly in the glass. The direction the light travels is determined by the phase front (the line of peak amplitude) of the light wave. (a plane wave is a stream of parallel phase fronts). The light will always travel forward, perpendicular to that line, but if one part of that beam is inside glass (say, at the interface), then that part of the phase front moves more slowly than the part that's still in air, steering the light beam accordingly.

Of course, even in the vacuum of space, light doesn't quite travel in a straight line.
Due to its wave nature, light diffracts, or spreads out.
As one example, if you pointed a typical laser at the moon, its light would spread out so much that by the time it reached the moon, it would cover most of its surface. With better engineering, we can make that spot size smaller, but not to the point of having the beam be the same width as when it left the laser.

 

[Orodruin]

the basic property of light is that ,it travels in a straight line

No, light follows a path of stationary optical length. This is related to the index of refraction by integration of the refractive index along the path and known as Fermat's principle. Snell's law of refraction (the one which describes how light is refracted between two media) follows directly from this principle.

 

[Nugatory]

If nothing interacts with the photon, conservation of momentum tells us it will travel (approximately) in a straight line.

Nothing wrong with the rest of your post... But introducing the word "photon" into this topic in this way can only mislead. @thecosmos123456 , if you can forget that you ever heard the word "photon" until you've thoroughly nailed the classical behavior of light, you'll be better off.