Why the light travels in straight line?
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because its the shortest path. light does not bend with gravity, spacetime bends with gravity, changing the shortest path.
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Light is not an object, therefore it can't stop moving.  It travels is all directions.
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Light can move in a few ways, all of which are rays.  Rays are striaght lines, and light can never curve (unless you are in a black hole, if that's the case....too bad for you) but it can be REFLECTED or REFRACTED.\n\nReflection is when a light ray bounces off of a surface and does so in predictable angles.  On a flat surface, the light relflects at an angle equal to that which struck the surface.  This is how a flat, or plane, mirror works (bathroom cabinet, dresser).  A rounded surface can magnify and distort an image by reflecting the light rays at different angles over its entire surface.  The law of reflection still holds true, but since the surface is irregular, the image reflected is not true to life (fun house mirrors, magnifying mirror for make-up or shaving).\n\nREFRACTION is the bending of light as it changes mediums and cause changes to images as well, such as when looking into an aquarium tank.  I know you didn't ask about this, but call it a bonus.
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No.\nA straight line is obtained by joining two distinct 'points'.\non this line there lies infinite 'points' between the given end-points.\nSo, a strsaight line is actually a set of POINTS.
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You must to review the diagram... note that the light escape in a second reflection, no in the first reflection.
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The way light particles travel is  in a ray sequencce at the same velocity.\nThe ray path light follows looks like traveling beads at the same spacing and travelig period. When the traveling period of the particles is shorter the frequency that  they cross at agiven point is higher. If the period is longer the frequency is lower.  It is at the lower frequency that our eyes see light radiation ray towardsred .This is what is called the red shift.  So light appears to follow the rules of a waves yet its not a wave.(I am not going to explain oscillatory motion in this answer.)\n\nIn the wave there are two types of effects; one is called the Doppler effect on which the assumption that the galaxies are receeding is made.\nThe other is called the Dispersion Effect.This case shows also a red shift but has nothing to do with the recession of the galaxies.\nInitially this was the belief of Einstein .On that basis he used a constant called the cosmological constant to account for the stability of the Universe. Later he reversed his belief and thought he had made a mistake.  HOwever the Dispersion theory showed other wise; Hence Einstein may have not been wrong in the first Place. Even Einstein was swayed to change his mind and believe wrong.\n\nIt is a matter of preference whether you choose which is the correct theory about recession of the galaxies.He did believe that the Universe is a finite Creation. That makes sense other wise motion could not take place and the Energy Phenomena would not exist.( I am not going to explain how the Energy Phenomena takes place in this answer)\n\n\nTo Understand the motion of galaxies it is necessary to know how much mass loss or gain is experienced by a paticular galaxy. If the galaxy increases in mass ,it will increase its radius from the center of mass of the Universe.If the galaxy looses mass the reverse occurs.This is my Theory. It is a Theory of Equilibrium for rotation. You can you choose to expand on it or believe the Gauges theories or quantum Mechanic theory.  But none offer this answer. The is quite  a shift and I doubt that the main stream of physics would believe me, because they believe their theory of the red shift is the only right one!
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yes there are straight lines. what you describe is according to the earth and the solar systems that appear round due to the force that acts from all directions creating a universal container (the sphere) which is round. But look at the BIGGER picture a straight line has to exist even if it is as small as an atom (universe - meter scale). also there are other geometrical forms. infact the only reason why round is soo common in all universal shapes (objects). well think of all forces acting on a body from all sides then reason the kind of shape it will produce when all forces level out the object.
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The same reason why a pinhole camera works.  As light passes therough the pinhole, it refracts and is focused.
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The question is too philisophical. We don't know WHY light travels so quickly, we just observe that it does. We can derive such a speed and we can derive the fact that it always travels at this speed no matter what your reference speed is. We have failed to find a situation where the speed of a single photon has not been the accepted speed of light.\n\nAlso, contrary to a previous post, light travels at a constant speed, independant of its frequency. This opposed to mechanical waves such as sound and water waves..
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As you know, the 'bending' of light changes its direction.\nSame thing happens to sounds - you can hear sounds from around a corner.
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What you are describing is a result of light reflecting off a concave surface.  The best way to explain this is like this:\n\nDraw a big T on a piece of paper.\nNow draw a line from the lower right side to the vertex of the T (that's the intersect).  This represents light coming in.\nNow draw another line from the vertex to a point at the same location as the origin of the other line, but on the lower left.  That is the light leaving.  The angles of both lines converging on the crossing of the T should be the same.  Same with any beam of light.  It is reflected at the same angle as it is received.\n\nA spoon is a reflecting concave mirror.  If you could get close enough to the spoon you could see your image right side up.  But when you are further out than the focal distance the image inverts because the beams cross over each other.
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when light moves from 1 medium to another..say air to water..it changes in its speed...this change in speed leads to a change in the direction of the light.
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There are two ways light can bend:\n\n1.  Refraction/Diffraction:\n\nWhen light travels from one medium to another, such as air to water, the light slows down (or speeds back up depending upon which medium it is entering and leaving).  This change in speed causes the light to bend.  The reason light slows down is because the photons interact with particles in the medium.  I show this in class by doing the following: \n\nI walk from one end of the room to the other without stopping.  That represents light going through a vacuum.  Then I walk the same path, but this time I shake the hand of each student I pass.  This represents light going through a medium such as air or water.  Obviously it took me longer to do the second trip.\n\nDiffraction is another way light can bend.  It is due to light bending   as it travels through a small hole or slit.  It also occurs when it bends around a corner, but that amount of bending is very small.  If you have ever been in a dark room, and the light is on in the hallway, no doubt you have noticed the light doesn't come in at a right angle from the door's opening.  It bends at a small angle from the door's edge.  That is diffraction.\n\n2.  Gravity:\n\nGravity can be thought of as the curvature of the fabric of spacetime.  If you put a bowling ball on a trampoline, it will create an indention.  The trampoline represents spacetime, and the bowling ball represents either a planet or the sun.  Light travels in a straight line through the spacetime fabric, much like if you were to draw a straight line on the flat surface of the trampoline.  However, if you were to draw that line through the indention of the trampoline, caused by the bowling ball, that line would curve.  So, it isn't that gravity pulls on the light.  The light is simply falling the contours of spacetime.
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think about looking at the horizon, its a line that runs from side to side , sky line in the horizon(horizontal line)
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Assuming theta is the angle the ray from the light to P makes with the vertical. Call the height x, then distance to light is sqrt(x^2+100), intensity = 1/(x^2+100) * x/sqrt(x^2+100), or x/(x^2+100)^1.5.\nFrom the ref., the derivative is\n1 / (x^2 + 100) ^ 1.5 - 3 * x^2 / (x^2 + 100)^2.5 (sparing myself the tedium of the chain rule.) Set the derivative = 0 and multiply by (x^2 + 100)^2.5, yielding x^2 + 100 - 3 * x^2 = 100 - 2 * x^2 = 0, so the derivative is 0 at x = sqrt(50) = sqrt(100*0.5) =10*sqrt(0.5). Thus the light should be at 7.07m height.
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Since the light beam actually transfers momentum to the reflector, I believe you can "consider" that the photons have slowed down and reversed direction.  But that is not what "really"  happens.  The time it takes for 10m in one direction and two 5m paths is the same, as the velocities are identical in both directions.  The momentum of a photon is not related to its velocity (as it is in physical objects); it is a function of its wavelength only.
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The property of light is that it takes the shortest route possible between 2 pts. IT IS A WRONG IDEA THAT THIS PATH IS A STR LINE. A str line is the shortest dist in Eucidean Goem, the geom of the Universe is NOT EUCLIDEAN IN NATURE. As for your other ideas, I pt out that a photon never stops travelling unless it gives up its energy. DO NOT USE NEWTON'S LAWS ON THE PROPAGATION OF LIGHT. THE PROPAGATION OF LIGHT IS EXPLAINED BY 2 THEORIES-\nMAXWELLS THEORY OF ELECTROMAGNETISM, speaking of the WAVE ASPECTS\nEISTEIN'S GENERAL THEORY OF RELATIVITY, speaking of the particle nature. GO through these when you r of the proper age to do so, let me warn you that studying the true nature of light requires a huge pre-knowledge of physics and it is very difficult and controversial.