question: How light can bounce off objects but it has no mass.
comment: Light does not "bounce." A photon of light is absorbed by an atom, which excites an electron in that atom. The electron then relaxes, which causes the atom to give off (or emit) a photon of light. Think of it more like a person catching and then throwing a ball, as compared to a ball bouncing off of a wall.
passages: [Newton's] physical reality is characterised by concepts of space, time, the material point and force (interaction between material points). Physical events are to be thought of as movements according to law of material points in space. The material point is the only representative of reality in so far as it is subject to change. The concept of the material point is obviously due to observable bodies; one conceived of the material point on the analogy of movable bodies by omitting characteristics of extension, form, spatial locality, and all their 'inner' qualities, retaining only inertia, translation, and the additional concept of force.
   Section::::Contributions to the mass of a system.
   Photon
  
  The photon is a type of elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force (even when static via virtual particles). The invariant mass of the photon is zero; it always moves at the speed of light within a vacuum.
   "The Catholic Encyclopedia" of 1913 describes the result as follows: 
   BULLET::::- a mirror will never reflect exactly the same amount of light cast upon it, though it may concentrate the light which is reflected into a narrower beam.
  
  BULLET::::- on hitting the ground, a ball previously in free-fall (meaning no force other than gravity acted upon it) will never bounce back up to the place where it first started to descend.
   Newton's corpuscular theory was an elaboration of his view of reality as interactions of material points through forces. Note Albert Einstein's description of Newton's conception of physical reality:
   The basic SI unit of mass is the kilogram (kg). In physics, mass is not the same as weight, even though mass is often determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
  
  Section::::Phenomena.
   The light orbital angular momentum of a particular photon can be any integer "N", including zero.
  
  Section::::Physical properties.:Experimental checks on photon mass.
   Objects on the surface of the Earth have weight, although sometimes this weight is difficult to measure. An example is a small object floating in water, which does not appear to have weight since it is buoyed by the water; but it is found to have its usual weight when it is added to water in a container which is entirely supported by and weighed on a scale. Thus, the "weightless object" floating in water actually transfers its weight to the bottom of the container (where the pressure increases). Similarly, a balloon has mass but may appear to have no weight or even negative weight, due to buoyancy in air. However the weight of the balloon and the gas inside it has merely been transferred to a large area of the Earth's surface, making the weight difficult to measure. The weight of a flying airplane is similarly distributed to the ground, but does not disappear. If the airplane is in level flight, the same weight-force is distributed to the surface of the Earth as when the plane was on the runway, but spread over a larger area.
   In his "Hypothesis of Light" of 1675, Newton posited the existence of the ether to transmit forces between particles. In 1704, Newton published "Opticks", in which he expounded his corpuscular theory of light. He considered light to be made up of extremely subtle corpuscles, that ordinary matter was made of grosser corpuscles and speculated that through a kind of alchemical transmutation "Are not gross Bodies and Light convertible into one another, ...and may not Bodies receive much of their Activity from the Particles of Light which enter their Composition?"
  
  Section::::Renaissance and Early Modern.:Diffractive optics.
   In physics, apparent weight is a property of objects that corresponds to how heavy an object is. The apparent weight of an object will differ from the weight of an object whenever the force of gravity acting on the object is not balanced by an equal but opposite normal force. By definition, the weight of an object is equal to the magnitude of the force of gravity acting on it. This means that even a "weightless" astronaut in low Earth orbit, with an apparent weight of zero, has almost the same weight as he would have while standing on the ground; this is due to the force of gravity in low Earth orbit and on the ground being almost the same.
   Mass
  
  Mass is both a property of a physical body and a measure of its resistance to acceleration (a change in its state of motion) when a net force is applied. An object's mass also determines the strength of its gravitational attraction to other bodies.
   The unit of measurement for weight is that of force, which in the International System of Units (SI) is the newton. For example, an object with a mass of one kilogram has a weight of about 9.8 newtons on the surface of the Earth, and about one-sixth as much on the Moon. Although weight and mass are scientifically distinct quantities, the terms are often confused with each other in everyday use (i.e. comparing and converting force weight in pounds to mass in kilograms and vice versa).
   A photon's momentum is a function of its energy, but it is not proportional to the velocity, which is always c.
  
  For an object at rest, the momentum p is zero, therefore
  
  The rest mass is only proportional to the total energy in the rest frame of the object.
  
  When the object is moving, the total energy is given by
   A better scientific definition of mass is its description as being composed of inertia, which is the resistance of an object being accelerated when acted on by an external force. Gravitational "weight" is the force created when a mass is acted upon by a gravitational field and the object is not allowed to free-fall, but is supported or retarded by a mechanical force, such as the surface of a planet. Such a force constitutes weight. This force can be added to by any other kind of force.
   Section::::Pre-Newtonian concepts.:Galilean free fall.
   In terms of physics, an object is said to be dark when it absorbs photons, causing it to appear dim compared to other objects. For example, matte black paint does not reflect much visible light and appears dark, whereas white paint reflects lots of light and appears bright. For more information see color. An object may appear dark, but it may be bright at a frequency that humans cannot perceive.
   In relativity, all the energy that moves with an object (that is, all the energy present in the object's rest frame) contributes to the total mass of the body, which measures how much it resists acceleration. Each bit of potential and kinetic energy makes a proportional contribution to the mass. As noted above, even if a box of ideal mirrors "contains" light, then the individually massless photons still contribute to the total mass of the box, by the amount of their energy divided by .
   of the Earth’s total mass. The self energy of the Moon is smaller yet, about of its mass. (The contribution for any object of laboratory size is negligible, about , so only measurements of planet-sized or bigger objects have any hope of seeing this effect.)
   For an object in free-fall, this force is unopposed and the net force on the object is its weight. For objects not in free-fall, the force of gravity is opposed by the reaction forces applied by their supports. For example, a person standing on the ground experiences zero net force, since a normal force (a reaction force) is exerted by the ground upward on the person that counterbalances his weight that is directed downward.
   Mass is (among other properties) an "inertial" property; that is, the tendency of an object to remain at constant velocity unless acted upon by an outside force. Under Sir Isaac Newton's -year-old laws of motion and an important formula that sprang from his work, an object with a mass, "m", of one kilogram accelerates, "a", at one meter per second per second (about one-tenth the acceleration due to earth's gravity) when acted upon by a force, "F", of one newton.
   Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar quantity, the magnitude of the gravitational force. Others define it as the magnitude of the reaction force exerted on a body by mechanisms that keep it in place: the weight is the quantity that is measured by, for example, a spring scale. Thus, in a state of free fall, the weight would be zero. In this sense of weight, terrestrial objects can be weightless: ignoring air resistance, the famous apple falling from the tree, on its way to meet the ground near Isaac Newton, would be weightless.
   For other situations, such as when objects are subjected to mechanical accelerations from forces other than the resistance of a planetary surface, the weight force is proportional to the mass of an object multiplied by the total acceleration away from free fall, which is called the proper acceleration. Through such mechanisms, objects in elevators, vehicles, centrifuges, and the like, may experience weight forces many times those caused by resistance to the effects of gravity on objects, resulting from planetary surfaces. In such cases, the generalized equation for weight "W" of an object is related to its mass "m" by the equation , where "a" is the proper acceleration of the object caused by all influences other than gravity. (Again, if gravity is the only influence, such as occurs when an object falls freely, its weight will be zero).
   Section::::Pre-Newtonian concepts.:Planetary motion.
   By defining the apparent weight of an object in terms of normal forces, one can capture this effect of the stress forces. A common definition is "the force the body exerts on whatever it rests on." 
  
presuppositions: Light bounces off objects. Light bounces off objects.
corrections: Atoms in objects absorb photons, and then emit new photons. Light does not bounce per se - it "bounces" off objects due to reactions between electrons and photons.
raw_presuppositions: Light bounces off objects. Light bounces off objects.
raw_corrections: Atoms in objects absorb photons, and then emit new photons. Light does not bounce per se - it "bounces" off objects due to reactions between electrons and photons.