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This issue came up in our game the other night. I made an on-the-spot adjudication, but I want to look into it further.

I've always worked on the assumption that by RAW light does not cast shadows.

The spell description notes that:

Completely covering the object with something opaque blocks the light.

The description does not address what partially covering the object does. On the assumption that a spell only does what it says it does, then a reasonable conclusion might be that partially blocking the light does not do anything. In other words, no shadows.

A counter-argument might be that it is only common sense that light from the cantrip casts shadows, because that's what light does, it casts shadows, and that it does not need to be spelled out in the definition, because it's the normal everyday interpretation of light.

A counter-counter argument would be, it's magic, it ain't physics.

So, my question is, does the light cantrip cast shadows?

Of course, the DM can interpret and/or make a house-rule. I'm interested in a RAW answer, and additionally, I'd be interested in what interpretation has worked for you.

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There is no rule which causes a partially obscured light to cast shadows.

...the DM, of course, can adjudicate otherwise for the sake of realism, because light sources create shadows in real life. There's just no actual rule mandating that people cast shadows.

According to Vision and Light (PHB 183), "shadows" is synonymous with dim light:

Dim light, also called shadows, creates a lightly obscured area.

Now, the light spell in particular creates such an area of dim light at the edge (PHB 255):

Until the spell ends, the object sheds bright light in a 20-foot radius and dim light for an additional 20 feet. The light can be colored as you like. Completely covering the object with something opaque blocks the light.

This description is almost identical to the mundane light source, the candle:

For 1 hour, a candle sheds bright light in a 5-foot radius and dim light for an additional 5 feet.

The lack of any specific rule differentiating magical light from mundane light leaves us with the only possibility that magical light illuminates the same way like normal light (unless otherwise specified; e.g. magical light can be dispelled). Simply, light creates light because a rule says it does, and it doesn't work any differently to nonmagical light because no rule says it does. (Remember also that light only causes an object to glow in way that emits light; it's technically not an area effect spell.)

But does a partially covered or blocked light create additional shadows? Certainly a device exists which allows a partially covered light source to emit light only in one direction: the bullseye lantern (PHB 152), while Darkness and Light (DMG 104-105) implies that opaque physical barriers block light as they would in the real world:

Darkness is the default condition inside and underground complex or in the interior of aboveground ruins, but an inhabited dungeon might have light sources. ... Bright light in an environment of total darkness can be visible for miles, though a clear line of sight over such a distance is rare underground.

But ultimately, there is no actual rule which says a light source casts shadows behind obstacles. Rules-as-written, if you're standing in a 10 foot room with a torch in the middle, the entire room has bright light even if someone is standing between you and the light, or if you're behind a pillar or some other obstacle. The DM can, as always, rule otherwise in the interests of realism, but it's not required.

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The light spell casts shadows to the same degree that any light source does.

Technically speaking, nothing in the game rules ever says shadows exist whatsoever. The rules for light producing items just say they throw a radius of light, and that light level exists everywhere in the radius. Nothing in the rules says that physical objects like dense foliage block light in any way; nothing says that objects that create total cover have a shadow; there isn't even a rule in the book that says that walls block light!

Because there doesn't need to be. This is clearly one of those areas where the players and DM are just expected to understand how light works in the real world, and apply that understanding to the game. We're meant to just understand that light, whether from a torch or a spell, doesn't go through walls to illuminate the corridor beyond, because that's not what light does.

So while nothing says the light spell throws shadows, nothing says a torch does either. This should not be taken to mean that light sources in the game are different from real life light sources in terms of their behavior.

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Yes, it casts shadows

You won't get a RAW answer much better than the wording of the spell you have already noted.

However I will point out that the wording for the light spell contains the identical text for the entry for torches:

providing bright light in a 20-foot radius and dim light for an additional 20 feet.

So it's reasonable to assume that is behaves in a similar manner to a torch.

You could consider looking at the Cover rules for what is means to obscure an object or provide cover from it. They say:

A target has total cover if it is completely concealed by an obstacle.

Therefore, any opaque object that can conceal the object affect by light provides total cover.

You can then look at the general spellcasting rules for Area of Effect. Technically, light doesn't have an area of effect, being a targeted spell with the effect to emit light. However this is the only rules on area of effect we have to go on.

A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover.

If you combine all the rules quoted with the rule you gave, it is safe to assume that RAI are that light casts shadows.

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